Using relative sizing and positioning (% values) rather than absolute (pixels).

Hi

I'm new to designing websites that match the WAI guide lines.  My
biggest problem with the guidelines is that you can't use absolute
sizing?

Does this really make a big different to the accessibility of the site.

I'm also looking for several different screen reads to try the site out
on any address of where and can download a trail copy or purchase them
from would be great?

Thanks

Daniel