I have to replace the cartridge bearings in my specalized from hub.

Anyone know what size replacement i'll need?

Its the one with the bolt through axel, rather than the usual quick release version.

I imagine there's umpteen varities of 20mm axled spesh front hubs. If you dissasemble the hub, the number should either be laser marked onto the bearing outer or embossed on the seal.

Possibly these, although read the small print at the bottom of the page about Stout hubs varying from year to year. They recommend getting the bearing code off the old bearing.

When you said "bolt through axle", I was presuming you have the DT Swiss 9mm bolt-thru skewers. If they're 20mm hubs, the bearing size may be 6804-2RS but, again, check the old ones before you buy.