Selecting surfaces

I'm having a bit of trouble with two things:

1. Having created a cylinder with a hole through it, I want to select
the flat face at each end. For cylinders in which this face is wide (a
large difference in diameter of inner and outer circle), there's no
problem either clicking on it with the mouse, or selecting it using
find. However, if I make a thin walled tube, the end surfaces can't be
selected with either the mouse or the search option. Is there a simple
way to solve this problem?

2. Having made a circle to extrude, there is a lower limit on the
extrusion depth (the diameter is 20m, whilst I want a depth of only
0.3mm).

I'd be grateful for any help.

Thanks,

Gwilym