A very good question, which is however easily answered.
the stirling started life as the S.31 (better known as the M4), a half scale plywood model powered by four 90hp engines (very cute by aircraft standards), her cheif designer did want a 112ft wingspan but the air ministry were having none of it, insisting on the 100ft still, which reduced range but also made the stirling manouverable enough to earn it the nicname "The Fighting Bomber", it also hugely increased the lake off and landing runs
It was found with using the S.31 that an extra 3 degrees of wing incidence would hugely help the problem, and so the landing gear was lenthened to give the extra 3 degrees, hance all stirlings had the long legs
this wasn't the only problem, the landing gear was too long to retract into the nacelles! so a two stage system was introduced, first the gear was taken virtically upwards, then backwards, the motors for this process bing inside the fusilage in case of a motor failure
this wasn't to be the stirling's last undercarriage problem though, the gear on the first full size prototype's first flight collapsed on landing! this was to do with the alloy used for the legs which was later replaced by steel, and the indercarriage problems were finally solved! so there you go, long legs= higher incidence..........