Clunk noise, and bottom bracket size
 

I'm a super novice so thanks in advance for any help :)

I have a 2011 Trek 1.1 with completely original parts. FSA Tempo 50/34 crank. Nothing changed out yet (except for the seat).
I have put about 1,400 miles on it, and while biking today, noticed that the left pedal would make a clunk when applying power. Sometimes it makes it on the downward stroke, sometimes on the upward. I removed the pedal from the crank and put it back on, checking to make sure it was tight.

I think either the bearings in the pedal are bad, or the bottom bracket is bad. But going bad at 1,400 miles seems early, no? I do ride this bike hard, though.
In any case -- can the pedal be disassembled and bearings repacked or overhauled?
How does one go about determine what replacement bottom bracket I should buy? There seem to be a gazillion styles and sizes, even just for regular square taper catridge style bottom brackets. What are the upgrade options to get a higher-end longer-lasting bottom bracket?
Thanks very much!

Based on your description,I suspect that the crank spline is worn and it's moving on the spindle. But stat by eliminating possibilities. The first step ithe easiest, eliminate the pedals by switching to another pair and seeing if anything changes.

Checking for crank arm play on the spindle is tougher. Sometimes you can confirm it by standing on your horizontal pedals, then sitting and gently turning them back a half turn and jumping again. Often this will produce a clear click or movement you can feel, in which case you've confirmed the cause.

Loose crank/spindle fit is impossible to fix, so if so a new crank is in order.