Constraints can come from unusal places
There's a popular myth that says that the size of the space shuttle was dictated by the limitations of carrying the shuttle over rail, specifically that there was a single tunnel (usually with a location given as Utah) that the booster would have to pass through.
The Truth
This is mostly an urban legend, though it's based in fact:
Transport by rail influenced (but did not dictate) the booster designs. The diameter of the shuttle boosters (12 feet) is larger than the largest train diameter (called load gauge), but we regularly ship things bigger than that, and have to dismantle things to do it. Tunnels cannot be dismantled, so there are hard limits, but it's hard to point to a specific tunnel. (source: stackExchange)