Tank size? My advice is the largest you have room for and can afford. I have found that if you start with one fish then you'll see another because the one you have seems lonely and needs company. Then you'll see another-you just have to have. Then another and before you know it your buying a larger tank after you just spent all that money and time to set up the first tank.
We started with one fish that my son brought home at the end of this last school year. Surprise, surprise!!!!...

That poor thing.... I knew NOTHING about fish accept that the old-school theory was to let the water set for 24hrs before adding any fish. Boy, did I have a lot to learn!!!!!! (and still do!) I put it in a 5 gal tank....knowing nothing about "cycling." Of course the poor little thing didn't make it.

Just before it died we had bought it a tank mate. That one lived longer as I found books and talked to every pet/fish store within a 30mile radious. That was still before I found this wonderful site.
We moved the surviving one to a 10gal...(we already had the 5 & ten gal tanks from other "pet" projects). Out to the store to buy a new hood, more gravel, a heater, etc. My son added 2 more fish We upped it to a 36 gal withmore costs for more supplies....and since then with 3
more fish (all goldfish) we are up to a 54 gal corner bow tank.
The 10gal now serves as a sick tank and the 36gal is now filled with different kinds of Tetras.
Kudos to you for educating yourself before you buy your fish. I wish I would have had more time than 2hrs notice that we were adopting a sweet little goldfish.
One more thing you might want to check out at your pet/fish store..... Really check out the display tanks. Find out which fish you want and ask how many of their display tanks are on the same filtration system. Most in my area have 6 tanks to a filter. Check out the fish in all the tanks on that system for obvious signs of diseases...ich, fin rot, etc. If there's one sick in one tank on that system, then it's probably filtering through the other tanks too. One of the fish I brought home (during the beginning of my aquatic education) gave the other fish in my tank Ich. No matter how hard I tried I still ended up losing my favorite to futher infections that escalated from the inital Ich episode.
Well, I hope I didn't bore

you too much with my story
Good Luck with your new adventure....