Here's teeny one. It's a small carpenter bee, Ceratina, on the flower of Centaurea. Look at all the white pollen on it's leg.
To show how tiny it is, here it is with a honey bee:
Another tiny one is this sweat bee, Halictus:
Here it is with some honey bees to compare with.
On the opposite side of the scale are the Bumble Bees. Here is a biggy, Bombus nevadensis, having trouble getting into Penstemon strictus otherwise known as Rocky Mountain penstemon.
A much smaller bumble bee is the brown belted bumble bee, Bombus griseocollis, here in my spotted gallica rose:
Here is a ground bee, Anthophora bomboides, with an amazing mouth:
This is a leaf cutter bee. I've seen plenty of it's workmanship on my roses, but hadn't seen the culprit until now. Lithurgus apicalis: