When you choose eyeglasses, what you should consider most is your face shape and clothes style not just the shape of your head. Usually, you shall buy eyeglasses that contrast to your face shape so as to complement your facial features. As i don't know what types of person you are, i can't give you an answer. Maybe you can try a pair of black nerd eyeglasses which popular and fits for all.
I think the big frame square eyeglasses may suit the round head. The square shape may make the round head look not that round. And the big frame type may leave the impression of calmness and coolness which round head can't get this effect. In a word, not choose the glasses with round lenses.
Under normal circumstance, if you have a round head you had better not choose the shape of a circular frame glasses. However, if you not, it would seem that your head is rounder. Therefore, you should avoid matching like this. However, you cannot choose the large extent contrasts with the head, because it would look obtrusive. As for the selection style of glasses decided by you, but you should remember the key to following with the principles above.