I'm looking to buy a Seymour Duncan pickup for the bridge position of my EC-256. I measured the width of all the strings at the bridge, and it was 52 mm across. Would I need a Trembucker for this, or would I need a standard?

Yeah, that is fender spacing, so you'd need trembuckers.
I don't give a shit if you listen to me or not
Make sure you are measuring from the center of each string. A traditional humbucker is 50mm from the center of each E string (not the outer edges). If your guitar has a trem, or if it is larger than 50mm, use trembuckers.
Dave @ Seymour Duncan
Quote by Tony Done
I've used G and F spaced pickups in assorted guitars and I don't think it makes the slightest difference provided that the string are somewhere over the poles. - You could adjust the pole height to compensate if you thought that it was necessary.

Agreed; I haven't really experimented with polepiece spacing but I've owned a Squier Jaguar which originally had shallow threaded saddles and knocking the string out of place there made next to no difference to tone, likewise when I snag the high E on my Telecaster with brass barrel saddles and knock that out of place.
Quote by H4T3BR33D3R
Youre officially uber shit now.

Quote by StewieSwan
3d9310rd is far more upset than i 
Does your 256 have a Floyd? If so buy the Trembucker. If it's a fixed bridge buy the regular. That's what ESP puts in them. . .

My fixed bridge E-II Eclipse came with a good 'ole SH-4 JB in the bridge.
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .