Ok.. There are no standard size for a bass drum. It all depends on the individuals.
The standard measurement for drums are in inches..
U can go to any Music store and U'll be able to see the different sizes of bass drums.. The most common being 22" and 20". There are bass drums that are 24" but its not that common.
For the positioning of your pedal to the center of the bass drums, U have to adjust ur pedal beater to ur desired position.
Usually, by default, if u were to position the pedal and fix it properly, the beater head will be at the center.. Unless U have different taste, U can readjust the beater head to any position that U desire..
Try it out.. Who knows, ur "dream" bass drum sound may not necessarily comes from the center...
Good luck!!! :lol: