Anybody has an idea of how to solve this problem?? I think it isn't very difficult, but I'm just not able to see this kind of problems

I've thought about brute force (calculating the area of all squares and taking the biggest one), but I don't know how to generate all possible maximum squares that can be formed from a point (low-left corner for example)!
Thanks!
PS: Anybody knows where can I find geometric/grid problems with their solutions? I don't know how to approach that kind of problems (Problem D from NWERC '05, 10167...)