I played on a 12 x 6 table for years. We played Empire III. We could fit three corps plus a cav corps per side. More than that and it got crowded.
Usually I have found that for 25mm to 28mm it was best to figure one corps (2-3 divs of inf, 1 cav and several batteries) per 4-6 feet of frontage for most battles. Compress it down and it just got too crowded.
The largest table I ever played on was at a game shop – it was probably 20 feet long and 4-6 feet wide. We had some real "mashes" on that one. It was mainly open to anyone that wanted to show up on a Saturday.
The largest surface I ever played on was an empty room we used for gaming at a friend's house. We had some large battles there but I cannot remember ever handling more than one corps plus some additional cavalry.
I am more into the experience than being able to say I maneuvered a lot of troops.
A friend of mine set up a battle where I had something like six divisions to move around – that was a bit too much.
paints his first three ships from the starter set.