What is the meaning of a circle on induction cooktop?
The most important part of it is to help decide where to put the pan. The second reason is to roughly decide what size pan will be OK on it. 

With the size, it is very small pans which cannot work. For domestic movable units, the minimum size tends to be 12 cm. If the pot is smaller than that, the cooktop won't work. 

The cooktop should always work if you place a too large pot on it. It is still not a good idea to do so, since you will get uneven heating of the food, and your pans will warp. But cookware with a sufficiently large ferromagnetic bottom (which might not be the full width of the bottom for sandwiched models) should in principle work.