Sun protective clothing interfering with bike mirror
This is complicated.  The farther from the eye a mirror is, the larger it must be to provide an adequate field of view.  This is one of the (several) reasons that bar-mounted mirrors are often unsatisfactory.

Assuming you stick with helmet-mounted, you do want to get the largest mirror you can find, and one with a relatively long arm, so you can get it as far out to the left as possible.  But I find that even without any helmet covering, this doesn't quite allow me to see around the helmet.

What I do is aim the mirror so that, when I'm looking straight ahead, the mirror gives me a view of the left shoulder of the road.  Then, when I want to observe traffic behind me, I turn my head slightly to the left.  This give me a view behind, and the turning of the head adds sufficient leftward extension to the mirror to allow me to clearly see past the helmet.