There is such a thing as too large
http://www.dudadiesel.com/img/items/HX115280_3013_L.jpg
(those are 2" ports; that monstrosity comes in a 290 lbs)
But it actually all comes down to your water flowrate. Audiedoggy's advice that you should always go longer and thinner for more efficiency is correct 98% of the time, and 99.99% of the time for home brewers. It is almost always the way to go - but that begs the question, so why then are the larger heat exchangers made?
Most homebrewers have tap water flowing at 5-10 gpm. A microbrewery might have 15 gpm tapped straight from the main, but often when breweries get bigger and bigger, instead of looking at increasing water flow, they simply get a glycol chiller, which is 2-6 gpm. At these low flowrates, you won't have any restriction no matter how few plates you have, and having a longer plate is more efficient than more plates, so for the money, that's the way to go.
However, if you were to get an industrial well like we did, and start pushing water at 80 gpm, then 20 plates will severely restrict your flow, and water flowrate is the bottleneck that everything is based around. So once you start talking high flowrates, that is when you want more plates in order to lower your pressure drop.
TL;DR 99% of the time a longer unit with low plate count is the most efficient you can go per $. In the extremely rare event that you have water flowing at 25+ gpm, let me know your water flow and temp and I can help run the numbers for a larger unit.
*edit
I forgot to add - yes, there is a problem with going too large if your flowrates can't support the number of plates. You can create a pooling effect where your fluids are moving at different speeds in the individual plates, which will decrease efficiency.
