Garmin 7 Series...

If you mean the Garmin Nuvi 7** Series, then yes, they'd fit in a jacket pocket but I'm not sure what satellite reception would be like. (I'm just about to nip across the road to the local Chinese restaurant, so I'll bung my 760 in my jacket pocket and see what sort of signal it gets.
However......by putting the GPS in a pocket and relying only on voice instruction, you're depriving yourself of much of the information it will provide, in terms of routing, mapping, speed cameras, etc. Far better to mount it on the bike so you can get the visual information as well. Nuvis aren't waterproof, apart from the
Nuvi 500, so that presents something of a challenge but it can be overcome.
The other issue that's often raised in relation to Nuvis is that it's not possible to create routes on the PC and upload them to the GPS unit. That's not quite true of the high-end models such as the 7** Series which support up to 10 routes. However, the units come with City Navigator NT pre-installed and aren't shipped with a copy of the software on disc. So if you wanted to have a copy of Mapsource running the maps on your computer, you'd have to source these in addition to purchasing the GPS unit.
Finally, if you do decide that you want to go for a Nuvi, you should look at
the whole range.
HTH