I've never used an Etsy gift card, so I'm not sure how they work. I know the shop you're buying from has to accept them for you to be able to use them, though.
I picked out my design for March. It's going to be pretty dang huge because of the detail. Not sure if I want to crayon tint it or use some other way of filling in, or if I just want to do colored thread.
Most patterns will give you the finished size of the project. If the finished size is 5"x7" you'll want to cut your aida at least a couple inches bigger than the finished size (so your aida should be at least 7"x9"). Keep in mind that if the pattern calls for 16 count aida and you want to use a different size, the size of the pattern will vary.
As far as following a pattern- if you get a kit make sure to sort your threads before you start. There are little cardboard cards you can get to keep the thread separate. This makes everything go much smoother! I also recommend getting a magnetic board for your pattern- it comes with magnetic strips that you can use to help you keep your place on the pattern. I highly recommend following a 10x10 section of the pattern at a time. You can block out the section on the pattern with the magnets, and it makes it much easier to follow.