The size of a painting determines the size of the mat. And it’s the size of the mat which determines the size of the frame. Don’t let the size of the frame control the size of the mat, or, God forbid, let it determine the size of your painting!
Depending on whether you want to go with a mat that’s uniform in width… or one that’s weighted at the bottom, you may end up using different-size frames for the same size painting.
Also, depending on whether a full sheet of your watercolor paper is 90#, 140# or 300#, the actual overall dimensions may be slightly different. If you had ordered a number of custom mats with predetermined openings, you’d be up the creek! I found out, for example, nominal full-size sheets (22×30) of Arches 300# paper are slightly larger than some other manufacturer’s lighter-weight paper. If the overlap for your somewhat larger paper all around the mat opening is only 1/4″, the opening in the mat designed for smaller paper would be too big for the other manufacturer’s smaller paper. You might want to take this into consideration when sizing openings for your mats. Some artists overlap their mats by as much as 1/2″ to give them a bit more flexibility. But realize, when you do this, less of your painting (1/2″ all the way around the perimeter) will show.
With respect to mat overlaps and paper sizes, if you hadn’t thought about this and were in a rush to get your painting matted, those precut mats you have on hand might not work…, particularly out of town, when participating in a plein-air festival. You’d be severely compromised in finding another mat with a slightly smaller opening!
Therefore, I strongly recommend measuring your paper precisely so you know what the precise dimensions are. Also, find out the actual dimensions of any other paper you might wish to use. The actual length and width of your paper are important, but also the thickness.
I prefer the traditional weighted bottom mat for larger paintings. That’s to say, I like the bottom border of a 4-sided mat to be wider than the other three. I generally prefer a white or off-white, single, 4-ply, archival mat. In cases where there’s lots of original white paper showing around the edges, a second mat of somewhat darker value can help define the original image.
I use archival foam board or a full-size 2-ply archival mat as a backer for my paintings. Secure the front mat to the backer and your painting to the 4-ply front mat with archival linen tape hinges (firstname.lastname@example.org), not masking tape. The number and length, as well as the configuration of the hinges, depends on the dimensions of the paper, but also the thickness of your paper. Use at least two hinges 2″ long across the top of 22″ wide paper, and three hinges for paper that’s 30″ wide. You may want to use additional hinges around the perimeter if the mat overlap is marginal. If not done correctly, your painting will come loose from its mat. You don’t want this to happen. Drop by your local framer so they can show you how to mount hinges correctly.
I make it a practice to place the title of the painting and my signature along the top edge of the mat’s bottom border. I don’t believe signatures on the front of the painting itself have anything to do with the composition of a painting, so I make it a practice to leave them off. Instead, I sign, date, and title my paintings on the back. However, once my painting is no longer exhibited in public, I’m happy to sign it on the front. For similar reasons, I’ll gladly sign the painting on the front if it’s a commissioned work, or if it’s more than 3 years old, since these paintings are rarely eligible for juried exhibits.