Repositioning shocks

[Goose51683]

Hey guys, I have been thinking about replacing my two rear shocks with on beefier shock that is located under the seat and mounts just ahead of the wheel. I've already removed the rear fender and have repositioned some of the fuse boxes etc under the seat. I think a shock could fit but I am unsure of what size shock would work for this. I know the length I would most likely need but I've never replaced a shock and I'm not sure if their is a weight requirement I would need to fulfill.

Any help would be greatly appreciated.