To add, the short piece length determines where the buckle position would be on your wrist. For your 7" wrist, a 90mm short strap will prob bring the buckle closer to the lugs, instead of center on your wrist. If you prefer a more "symmetrical" look instead (with buckle in the center of your wrist), you prob need a 75mm or 80mm short piece length. This is personal preference -- for instance, many Panerai wearer likes to see the buckle (many uses rare, historic, or unique "pig" buckles), so would prefer this buckle position, as you'll see it each time you look at the watch.
The long piece length determines how much "tail" you have. Usually 115mm is standard. I like LONG tail which comes close to the opposite lugs of the watch, and so uses 150mm or even longer. Obviously type & thickness of the straps will also affect the effects of these lengths on your wrist. Thinner n dressier straps (like some crocs n snakes skin) generally look better with more conservative lengths (80/120 maybe), Calfs with medium lengths (80/130), and thick ammo straps look better with the longer lengths (90/150) -- (again, these are lengths for "standard" wrists of about 7").
For my 6-3/4" wrist, a 90/170 ammo strap allows me to see the dial, the tail, and the buckle -- all at the same time with one glance.
Hope this helps
