when I was young, strong, healthy, and thought nothing of lifting it, a 10" H & V was my first choice. I still think it was a good choice for the work I mostly do, and this is where you may decide to compromise. If you do small work, an 8" table is much lighter and more convenient to set up. Stood vertical, it interferes less with the spindle to reach center. I don't think a 6" is a good choice unless you know you will only be working miniature stuff. You use a considerable percentage of a small table just for set ups and hold down clamps on a lot of work, and a small table does not leave much room.
I like 90:1 gear ratio for circular milling, which is presumably your primary interest with a rotary table. My 10" table is 90:1, and that is a pretty good ratio for milling diameters (hand cranking) out to 12" or so. My 20" table is also only 90:1, and out near or past the rim (I do one part at 32" diameter) that always seems _very_ coarse.
If you plan to use the table for any indexing and dividing work at all, get one that includes a set of dividing plates, index pin, and sectors.
If most of your work is small, round to begin with, and will actually be dividing work, where you index the table, lock it, and then drill a hole or mill a slot with the machine axis drive, you might find a dividing head more useful. For instance, I've made gears on a vertical rotary table, and in a spin index. On the table, you need an insert collet chuck or other arrangement in the center hole to get the work out far enough to clear the cutter. It's a lot more convenient on a dividing head. OTOH, I don't find it fun or convenient to do much circular milling on a dividing head if the work diameter is much over a few inches.
Dividing heads typically are 40:1, so faster to index, position to positon. But the milling capability on a radius is limited by the "coarse" ratio to smaller diameters, as is the usual work holding (collet or chuck) arrangement. A dividing head will tilt from below horizontal to past vertical, so you can mill, drill, bore or shape profiles at any angle in between.
an 8" H & V with dividing plates and set up, can usually be arranged somehow to do most of the work you might want to do on it. It is relatively light to move, and convenient to set up with reasonable space (spindle clearance, e.g.)limitations to be considered. A little bigger (10") is better, if you will ever need the capacity.
smt