[url=http://halo.wikia.com/wiki/UNSC_Military_Units#UNSC_Marine_Corps]3rd MEU looks good.[/url]

Quick rundown on Spartan production vs ODST ranks:
1 - 300
1.1 - ~7
2 - Less than 75, Less than 16 survived the Fall of Reach
3 - 1200+
4 - ~500

ODST - 'several divisions' (at least 3 are named), 20,000 members each

Since Halo loves Spartans, and treats everything else as mook-redshirts unless scripted otherwise... records of entire military strength is hard if not impossible to determine.
-Usually as big as the difficulty level demands.

On top of that, the organization is... messy.