There is no proper way of telling that a certain length is ideal and the other is not ! but in general,the usual length for the is 1.5 to 3meters (as this will give a distance of 5 feet to 10 from the center where the usually the equipment sit ). Also usually the recommended minimum distance between the fronts is 6 Feet (from right to left) !
So this should be the ideal,but then again it also depends on the room size and other things ! but in any case try not to exceed the max length for two reasons,one two wide the speakers are the stereo image can get messed sometimes ! second,if you don't use the proper cable there can be lot of interference due to long distance not only in terms of electrical and stuff but also in terms of Inductance and Capacitance that the cable will add !!
Now coming to the Gauge ! Yes it is infact called Gauge !! its the measurement term used for measuring the thickness of the cable,its also measured in Square MM !! There are two standards used internationally,One is "AWG" or American wire Gauge (Introduced by americans to the world) the other is "SWG" Standard wire Gauge (as always the Europeans and British in particular) !! Now there are many things to these two,but I'll not bore you all the page long explanation ! here's a short and easy explanation !
The difference between the AWG and SWG are the same as the rest of the things thats there between the US and Europe !! For Ex. the weather,use of decimel etc etc..
Finally the proper Gauge for the fronts !! I usually say 12-14 for the fronts and 16-18 for the rears ! so thats the one you should keep in mind !
Hope this helps !!