A micro is anything film canister sized and smaller. The smaller they get the more difficulty they can be because of the needle in the haystack effect. More spots in the same space are available the smaller they get. The smallest that I've ever found was the size of the eraser on a pencil.
Based on your post, two spots I'd of looked are:
The lamp post has a skirt as it's base that lifts up. A popular spot.
A chain link fence has caps on the posts. Some are loose.
Personally I find there are now so many caches I can skip micros and still keep my caching calender too full to get to them all.
Thank you for the suggestions. There was a metal sqaure piece that did lift up. We felt up inside it looking for something magnetic, but our fingers only found rust. There was a piece off the lightpost and you could reach inside, but wires were exposed and my husband thought it was an accident waiting to happen and shouted at our daughter to stop before her hand got in.
I'll try the caps idea. I looked on the backside of the posts, I thought it might be stuck there where you just had to look over, but no dice.
I'm in a rural area, and for some reason 80% of the caches within easy driving distance are micros. We were trying to hit the local ones first where we knew the streets and neighborhoods. It's really fun when you can find something!