It all depends on who you talk to, and where the tree is located. I've seen lots of timber buyers say a 14" red oak is "too big" and wipe it out during the harvest.
In a woods situation, too big gets to be when the tree starts to decline. Swollen knots are a sign that it's past too big.
Trees will continue to put on value, and lots of it, as they grow larger. But, economically, will slow down their rate of return after about 18". That doesn't mean they are economically or physcially mature. You will make more money on $100 @ 2% than you will $10 @ 10%.
From an equipment standpoint, most large mills can handle up to 42". I did see a carriage with an 8' opening. The economy of milling a log usually has a maximum of about 28". Above that, it takes too long to mill the log and production costs begin to go up. A lot will depend on mill setups and cutting patterns.
In an urban or homesite, trees have to have sufficient room for root and crown expansion. They may be too big if they would reach a building if it fell down. Usually, it takes quite a bit of decline, or a massive wind to bring them down. In your area, ice may also be a problem.
