For what it's worth a lot of the Kitfox fleet (33') here often wish theirs were shorter and sometimes some of Avid Speedwing (24') types wish we had a bit more.
I guess it depends on what you want or what you plan on doing most of. The tip extension kit method is worth considering as in theory it gives you the best of all worlds.
Having flown most of the standard types and most of the hybrid mixtures a couple of things stand out. A clipped Mk4 1200 (29.5') powered by a 582 gives the following test results at 1050 lbs AUW, unstick 38 kt, ROC 570 fpm, Best L/D speed 50 kt at 600 fpm descent. Clean Stall power off, buffet 30 kt, break 28 kt Half flap power off stall the same figures, Full flap power off stall no break and level sink at 20kt indicated (pitot error). With slight power full stall at 18 kt. Cruise performance 4500rpm/53 kt, 5000rpm/65kt, 5500rpm/75kt, 6000rpm/80k 6500rpm/89kt and 6800rpm/97kt (max level speed) Vapp 50kt slowing to Vthreshold at 40kt. The low speed figures need careful interpretation because of pitot error at high alphas. 1050lb is the UK maximum for the 1200.
Compared with a standard Mk3 same power this indicates an improvement in speed of around 13-15%. Landing float was not so apparant compared with the full span models, something that always scares me if it's a bit crosswindy.
Compare the above with the Avid Speedwing with a mere 24' span and the same power at 1045 lbs. Unstick 43kt, ROC 525 fpm@60kt, Best L/D speed 61kt at 700 fpm descent. Clean Stall power off, buffet 47kt, break 44kt. Half flap power off stall again similar figures 46/43kt, Full flap power off, buffet 42 kt, nodding break 38kt. Cruise performance 4500rpm/55kt, 5000rpm/67kt, 5500rpm/78kt, 6000rpm/90kt and 6500rpm/97kt and 6800rpm/102kt (max level speed). Vapp 62kt slowing to Vthreshold 57kt. We're limited to 1020lb MAUW to keep agreement with our 44kt stall speed.
What this shows is that there is really only about a 5% increase in speed over the Mk4 clipped wing and definately no float whatsoever, the wheels contact and you brake straightaway. As a result the landing distance is not disimilar to a 'fox but quite a bit hotter.
A 27.5' Speedwing came up with figures not disimilar to the 1200 though it was built excessively heavy and was not very clean (aerodynamically) nor well rigged
Though I haven't flown it there is a 30' span Mk3 here which has had the concave underwing form filled to level and the tips removed and simply sheared off. The plane was properly tested at gross weight and the following were obtained: ROC 845 fpm, Cruise 5800rpm/81kt, 6000rpm/86kt. Clean Stall 35kt, 15 deg flap stall 33kt. I have even got the conversion schedule if anyone's interested.
My bones tell me that 33' is just too long for anything but fun flying, you get good STOL performance but long range cruising becomes tedious. At 29.5-30' things appear to improve without too much been lost in the landing/taking off department. I don't think you can go a lot lower than this simply because 'foxes tend on average to be built heavier than Avid's. We can get away with 24' as long as the empty weight is kept around 530lbs but once it creeps up handling suffers at the low end. I've flown some of the German 912 equipped Speedwings and though they are impressive at the top they are a right handful to land.
At 29.5' with a bit more oomph, say 90hp then you should in theory see at a credible increase in top speed to around 107kt assuming we live in a linear world though it depends on how much extra weight is implied in developing that power. I have crude rule of thumb that says for each pound added knock 5 fpm of the climb rate.
Finally of all the accidents here the vast majority occurred at slow speed and near the ground or in one case water. Slow landing speeds are fine so long as a) you don't end up on the wrong side of the power/drag curve (very easy) and b) that the cross wind limits are observed. Having seen people grind the tips on landing because they hadn't reasoned that the cross wind component was probably only half their landing speed makes it fairly easy to see why some of the incidents have occurred. My other rule of thumb, if the crosswind component exceeds 25% of your stall speed find somewhere else to land or be prepared for some fun.