Transmission oil: How full is too full?

[Meanwhile]

My truck has a 10 speed automatic in it and there's a small spin on oil filter on the back. I've changed this filter 3-4 times now and when I check the oil level there's always oil at the fill plug running out. I didn't think much of it because my driveway isn't close to being level. The other day I noticed a puddle of oil under the truck, about pie tin size under the transmission. I took the truck to a small repair shop down the street from where I was and it turned out to be one of the lines for the cooler was leaking. They made up a new line, put it on, checked the level and the guy said it was full. I guess I'm having trouble understanding how it could still be full. Talking to the shop owner, I remembered reading that someone here had a problem with the trans cooler leaking antifreeze into the transmission. I asked if the this may be whats going on with mine and was told no, that mine doesn't have any coolant lines running to it. How over-full is too full and can it do any damage being over-full?

Maybe I got lucky and caught this early into the leak?