What gauge strings should i get for my 7 string for low tuning (G or F)

[d00d00]

I want to try out some lower tunings and it's about time for some new strings, and apparently thicker strings hold up better for that. Any suggestions? I've never bought new strings before so i'm a noob when it comes to this stuff

 

[muckypup1]

I would have though a 72 would be heavy enough.