What fork size is 32mm (inside diameter on the frame) considered? Good day, I'm new to the whole bike building process, but I've finally decided to make the step and build a semi-nice bike.

I've bought a really nice aluminium frame 2nd hand, but its sizing has me stumped. The inside diameter of the front part of the frame (where the front fork goes in) is ~32mm. I've been searching online (e.g. http://www.sheldonbrown.com/cribsheet-headsets.html) for a couple of days, but I cannot understand what type of fork I should get.

Should I get an 1 1/8", 1.5" or some other type of fork?

EDIT: After some investigation, the bike frame is an Alpinestar Almega DX, the same as in this picture  I couldn't find any specs though :( 

Here is what I measured, when I said it was ~32mms (apparently it's the headset, not the pipe itself). The other end is the same size. 

Thanks!
If you have a 32mm approximate head tube inside diameter, this is an 1 1/8" frame, assuming it is a standard headset design. 

An 1 1/4" head tube would have approximately a 35mm inside diameter. 



Here is a link to a PDF from Cane Creek detailing most other styles of headsets fitment for non-traditional options. 

These have become very common also.