There are many variables that can help decide the right size of a watch, but a quick rule of thumb "calculator" that works very well for me is to divide your wrist measurement (in mm) by 5 for your smallest case diameter that will work, and by 4 for your maximum case diameter.
I have a 6.3" wrist, which is 16.002cm, or 160.02mm. Dividing that by 5 gives me 32mm for smallest case diameter that will work for my wrist, and dividing by 4 gives me 40mm as the largest. The middle point of that range (32-40) is the "sweet spot" which is 36mm, so ideal for me are things like the 36mm Rolex Datejust or Explorer, Eddie's Timefactors 36mm range, and the first generation Omega Aqua Terra's in 36mm (I have the quartz models).
For G-Shocks there are really only 3 that work for a small wrist. The squares are the best choice, followed by the Casioak, which is big but the slim height makes it wearable, and I also have the analogue G100.
Having said all that, if you have a 7" wrist, that really isn't small. That's bang in the middle of a normal distribution of male wrist sizes. The wearable range based on the rule of thumb ratios above would give you 36-44/45mm.
I remember a facebook poll in one of the watch groups that 149 people responded to, and which gave the following distribution when I charted it in Excel :
Mode 6.75, Median 7.00, Average 7.07