
(Above is an example of a dimensional card - note the button and badge and brad. Card is made with Basic Grey Serenade.) Card size is 4.75 x 5 inches.
The handmade envelope is a natural extension of the handmade card.
Today, when I create a card base I am unrestricted to the size - it is an unnecessary parameter. I am free to create whatever unique size my idea demands, as the envelope is as custom and unique as the card.
At artisan shows in my area my handmade cards in unique sizes with handmade envelopes sell faster than those cards with machine-made envelopes and card bases conforming to standard sizes. The unique cards and envelopes are also more expensive - but customers seem to demand the unique and are willing to pay for it.
The most common line I hear from folks purchasing handmade cards is 'she won't receive two like this one!' - probably stemming from the customer remembering an occasion in which they received two identical, store-bought cards...
Bubble envelopes: For delicate or extensively dimensional handmade cards I usually create a custom bubble-lined envelope, and the pattern I follow is from this post. Instead of liquid glue mentioned in the post I use an ATG gun for all but the envelope closure, which speeds the process considerably. Usually I plan to sell or save the bubble envelope and seal closed at a later date, so I use Scor-Tape at the closure and leave the protective lining in place. If I intend to insert the card and mail immediately I simply use the ATG and seal.
Handmade envelopes: One can try this method - not very professional in finished appearance but it works. I personally would never use this method to house a handmade card - it is not neat or 'clean' enough in appearance.
This method works well for very large cards.
Below is how I make most of my envelopes and the method below is my preference. Also I'll reveal my techniques, tips and tricks for speed.
I use a Martha Stewart Scoring Board with the triangle accessory in place.

Tools/materials: the handmade card that needs an envelope, large square punch or sharp scissors, corner rounder, envelope gum (or Scor-Tape or similar product), and cardstock of choice for envelope.

Measure finished card and add a minimum of 1/4 inch (1/8 inch each side) in height and width. For cards with embellishments that have excessive depth - add more than 1/4 inch - like an inch more (1/2 inch each side). For this card I created an envelope with finished measurements 6-3/4 inches in width and 4-3/4 inches in height.
For cards that are over six inches in width I generally use a 9 or 9-1/4 inch square (12-3/4 inches and 13 inches on the diagonal, respectively) of cardstock for the envelope. Anything under six inches in width I use an 8-1/2 inch square (12 inches on the diagonal) - I'll demonstrate:

The above red cardstock square is 8-1/2 inches square, which measures 12 inches on the diagonal. This 8.5 inch square is too small for this size card - not enough overlap.

This 9 inch sized square for the envelope is adequate. (note: For speed in doing math I usually cut the square to 9-1/4 inches...giving me 13 inches to work with when doing the math, versus 12.75 inches with a 9 inch square.) Perfect.

Now we do the math - You'll need the diagonal measurement of the square you are using for the envelope... in this case our 9 inch square is 12-3/4 inches on the diagonal. Subtract the finished envelope width needed (not the finished card width) from the diagonal, then the finished envelope height needed (not the finished card height) from the diagonal, then divide both numbers in half.
These two numbers are where the cardstock square is scored.

Note the triangle insert on the scoring board. For this envelope we are scoring at 3-1/8 inch, turn cardstock square a quarter turn, then score at 4-1/8 inch. Turn a quarter turn and score at 3-1/8 inch, turn another quarter turn and score at 4-1/8 inch.
side note: I usually calcuate the measurement of the second score line so I score twice, turn the cardstock square, score twice then done - time saver technique. I also jot down the envelope size and size cardstock square I used (not the finished card size) for use next time I create that particular envelope size, and accumulate these calculations on a designated notepad, and store it near my work station.

To speed things along, I use the square punch to remove the four notches formed at each side of the cardstock square. Often for speed I bend (not fold) the cardstock square from corner to opposite corner and punch two sides at once.
Or you could use scissors and cut out the notches, removing them.

I use a corner rounder on the top flap and the bottom flap. The side flaps won't be seen, so I don't waste my time rounding them. Again, I often bend (not fold) and meet the top and bottom corners and punch once through both layers - for speed.

Apply adhesive (I use an ATG) to secure the bottom panel.

Apply Envelope Gum and allow to dry if sealing at a later date. I use a scrap length of cardstock to mask off the top flap to form a clean, crisp edge of glue.

If the envelope prohibits a legible address from being written, provide a white adhesive label panel - I cut a full 8.5 x 11 Avery sticky sheet down to eight sections each measuring 2-1/8 x 2-3/4 inches, to avoid waste, and keep them in a clear bag near my work station.
side note: I do not adhere these white labels to bubble envelopes - instead I clip them neatly inside the bubble envelope. Why? It is quite difficult to write on cardstock that has bubble wrap under it. Let them write the address on the label, then adhere.

For standard, cardstock envelopes: for added protection, I frequently create an insert using a decorative piece of cardstock cut to the same size as the card base, and on the obverse I'll adhere a 4-inch square of corrugated cardstock.

The corrugated cardstock comes in 12x12 sheets - I prefer to cut a sheet down into nine squares each measuring four inches square, and store them near the white labels. Zero waste for the 12x12 corrugated sheets with this method. This additional padding will preserve the card's embellishments from postal sorters.
Hope you found this helpful, and hope you will try to incorporate some of the speed techniques I've mentioned the next time you create an envelope.