As a drummer mainly into rock music, the ideal BD size would probably be at least 24" BD. I would like that myself.
On the other hand, I like my toms to be as flat as possible.
The bigger the BD, the more angled toms you get.
If you don't like a "huge" sound, or like a jazzy sound, I guess 20" is OK, but then again; It could just as easy go down to 18" BD, if that's what you prefer.
I have a 20"x18" kick and I love it. It's a tighter more focused kick and I get people complimenting the sound of it. Since it's 18" deep, I get some good lows out of it, but I also get a lot of punch. Another good thing is that it doesn't take up a lot of space and my toms can get as low as I want them to be. You don't get the boom when you are sitting behind it like a 22" but when that put a mic in that puppy, the sound is awesome. I would rather have a 20" anyday! I also remember when I had my pearls, I heard a drummer playin a 20" kick and was like, that's the next kick I'm gettin!
i currently play a 22in bass drum. i have played 24x18. 20x8. and 22x16. the 22x16 is my current. which i love. but it lacks certain things i cant describe. im preparing to buy a Tama Starclassic Birch with a 20x18. and im thinkin that will be a sick drum. i like my friends maple pearl master customs mmx20x18 and i prefer birch. so i imagin the 20x18 will sound sick.
alot of factors really but i play 2 20 inch Rogers bass drums and i will compared both loudness and tone with anyone ..the key is the shells,the heads used,the beater,and most important the bearing edge and it's angle i prefere the 90 degree angle to the 45..it is easier to tune and centers the head on the drum..