If you have an unfinished attic go up and use your roof rafters as a guide

Here are pictures of my skylights i want to replace - are they fixed or curb mounted?

Yes they are curb mounted and fixed, depending upon where you live look at Insuladome or Velex curb mounted which was the old Thermo Vu brand on Long island that they purchased. Mease ure the inside of the box(inside opening) and from what they appear they will end being a size of 22 1/2" x 45" i do believe. Most of the standard curb mounted skylites were made to fit between 2 roof rafters 24" on center.

So I am just replacing the skylight that sits on that metal curb? shouldn't I replace the curb as well? it looks very old.

Yes most definitely i was telling you how and what to measure the curb does not count toward the unit dimensions