It also depends on what you plan to use this pad for. Are you just using it to practice in private as you assemble your act? Will you be using it in front of an audience? Will that audience be "coming to you" i.e., sitting around a table to see you or will you be "going to them" i.e., like table hopping?
My point is to take a look at what you do and where you'll perform and get or make a pad accordingly. I actually have several close-up pads for all these circumstances. Sometimes you want a big surface (like when you're performing as the star at the table like in L&L videos), other times you want a small surface like for table hopping (like the area of a spotlight pad.)
When performing the pad is your stage so it should look professionally appropriate for your act without drawing attention to itself or arousing suspicions as an item or prop (it should only draw the spectator's focus to what you're doing on top of it.) That's not to say it has to be made from the Golden Fleece ... I could easily see a "hobo magician" character unrolling a tattered towel and performing on that.
If you're just using it for practice in private, go as big as you can make/buy and afford to make/buy - give yourself room to work (unless of course you already know that your act must fit inside a 9-inch circle then of course target that.) Take a look at what is on the market in terms of close-up pads and model your personally-manufactured version after those you like (or tried if you have a magic shop within reasonable distance for you.) My personal preference are for pads with a little "give" to them. A piece of felt over a board would be too stiff for me. OTOH, I don't want to feel like I'm performing on a pillow. Many years ago I got the chance to perform on an authentic casino-grade blackjack (21) table in a person's home. It was padded perfectly for my tastes. I've strived for that type of feel ever since in my close-up pads.
So it will take some experimentation on your part. Your first pad may be too stiff/thin or too thick/padded. But you'll find out as you work with them regardless if they're commercial pads or of your own design.