I have 3 - 29 gallon tanks. . I think it's a really nice size for a beginner because you really have room to do a little real landscaping and have a decent number and variety of fish. Smaller tanks are just too limiting. I bought these mainly because I had several sturdy dressers I could put them on that were the perfect size and the drawers could store all the fish supplies. I didn't really like the look of the stands in the stores - they didn't look strong or steady enough to support the weight. My problem is that all but one of the dressers is fairly high so I have to use a little stepstool to pick out snails, take dead plant matter out of intake tubes etc. So I love the one on the shorter dresser where I can easily see and reach everything. I always have to roll up my sleeves to my shoulders though or I have a very wet shirt. I'm pretty short.
I would LOVE to get a 55 or 75 gallon tank but being disabled I think I'm in over my head with all the weekly water changes on the tanks I have so far. I can't imagine doing a 75% water change on a tank bigger than what I have.- oh and to have a 100-200 gallon tank would be so amazing - think of the kinds of fish you could have. But I rent an older duplex and I doubt my landlord would appreciate a tank exploding on the hardwood floors or falling through the floor. I could put a large one in the basement but even though I have the basement cleaned and sprayed regularly there are too many bugs and some gigantic spiders down there (obvious foundation issues letting little critters get in) - luckily little or no water gets in. Plus I don't handle stairs well any more. If you look at all the youtube videos of the bigtime amatuer fish keepers they always have their tanks in their basements. They are not particularly attractive tanks - since they are usually in it for the breeding - which I have no interest in.
Water changes have been REALLY good exercise for me - I'm building muscles in my arms again - I was such a depressed couch potato before these aquariums came into my life - they force me to function no matter how bad I feel and some days it's a real struggle but they are true miracle workers.
Sorry I ramble on - the question was best size of tank - and I'll say 29 gallons. I am fond of my cute little 5 gallon hospital tank too but wouldn't regularly keep anything but a sick fish in it - just too small.