The easy way to tell the sex is the more colorful and larger tail and body of the male. The better way is to look at the anal fin (the one just before the tail on the under-side of the body) - if it looks like a long tube it's a male. If it looks fan-shaped like an ordinary fin, it's a female. Just ask at your store - they should be able to tell you but check anyway. Guppies (especialy the males) are about the size of a tetra anyway so don't worry. I wouldn't suggest anything larger anyway as a 5 gallon can't hold anything larger. BUT, you shouldn't have fish deaths all the time anyway. If you read that link in my signature and make sure you give the fish the best possible home, they'll live a good 2 years.