So, there is actually a notation that isn't as well known with Roman Numerals. The Romans would typically write out all of the M's that were needed to do higher order numbers, however, when following the rules that were stated above, and when getting to VERY high numbers they actually had a notation that they used.
1000=M
2000=MM
3000=MMM
Now, you actually put a bar over the numbers, to represent multiplication by 1,000:
4000 is actually written as:
__
IV
So, then 5000:
_
V
5150:
_
VCL
And 100,000:
_
C
and 1,000,000:
_
M
And to go higher than that, you simply put 2 bars over the numbers to represent multiplying by 1,000,000.
1,000,000,000 would then be:
=
M
So, to represent their 6000 men, would be
__
VI or if they weren't being lazy and decided to disregard their own rules: MMMMMM
Hope this helps.