The 11” coil is a good balance between being able to achieve depth, weight and a reasonable factor of maneuverability. Multi vs single freq doesn’t have anything to do with coil shape, but the coil must be SYMMETRICAL from side to side and front to rear....which means an oval shape or a round shape. It can be a hexagon, a rectangle, etc....but round and oval are the easiest to wind and to use, along with the fact they look the best that way. A coils WIDTH, in general, equates to the depth it can achieve on a coin sized target in average conditions. The smaller oval shaped coils are more about being able to get into tighter spots(corn rows for example) while the huge ovals(CTX 17x13 for example) are all about ground coverage and more solid ID on deeper targets. Coils wound to a specific frequency might be a bit(or a lot) better at that specific task, but are obviously limited to that freq and the type of target it might encounter. BBS,FBS,FBS2 and Multi-IQ are VERY good at EVERYTHING....which is what makes them so desirable to such a wide range of users.