I purchased a tranny cooler for my A340H. Its a Hayden and its dimensions are 5"x12 3/4" x 3/4". I am now having second thoughts and wondering if it is big enough. They had another unit that was 7 1/2"x15 1/2"x 3/4" and one that was 10"x15 1/2"x3/4". Anyone running one of these or something similar? I was really tempted to get the big one but not too sure if its overkill or if it would cause problems. Any info/ links/ experiences would be very helpful before I get into this. I want something more than just running the fluid through the bottom of the radiator.

Yeah that's a little on the small side. I say get the biggest one you can fit in.

Give this a read. Linkie

I have the same Hayden #679 installed on my Dolphin MH with A43D automatic transmission. I generate losts of heat with weight, headwinds and a torque converter that does not lock up. I have a gauge with sensor on out line too cooler (where fluid is at highest temp) and I am very pleased how well the Hayden works in keeping transmission in FSM specified temp range.