When people say the size of of a turkey, are they supposed to say a 10 pound turkey, or a 10 pounds turkey? Even tho it's plural, it doesn't sound right to add an s.

If a number and a noun modify another noun, the first noun is almost always singular:

300 page book
100 metre race
two hour lesson
million man army

It used to be that any noun modifier was singular...

toothbrush (not teethbrush)
shoe store
stamp collection

...because a noun modifier is a noun pretending to be an adjective, and we don't have plural adjectives (we don't say 'beautifuls flowers', for example).

Particularly in newer combinations, however, plural noun modifiers are acceptable:

Watergate tapes scandal
systems analyst
complaints desk

OH, thank you, NNN.