will this kit work on the fabric side of an air mattress?

The flocking on an Intex air mattress can be easily removed by performing the following: Apply enough rubbing alcohol to the area to saturate the flocking (this will disolve the flocking adhesive). You can then scrape the flocking off with a dull knife-edge or scraper (I prefer a letter opener - sharp but not so sharp that you're at risk of cutting the material). Clean-off any remaining flocking adhesive with a paper towel or cotton swaps soaked with alcohol. Let the alcohol dry completely prior to applying the patch.

I'm worried about the top of the bed getting stuck to the bottom. How do you keep the adhesive from leaking through?

There's no way to keep it from leaking through but it dries fairly quickly. We just left our mattress inflated for a few days before we put it away.

What are the dimensions of the patch? It looks like it says 21 sq. in. but I'm wondering the actual dimensions. Can it be used on a large hole?

It comes as a roll about 5 inches long. I suppose you could overlap strips if the whole is very big. I can't imagine how it would work for a very big hole. Good luck.