Ok your post doesn't make sense. First you ask about 2500M not being enough then you ask about 2.5GB being the max.
The tutorial does indeed suggest that 2.5GB is the max "primary partition of no more than 2.5 GB". The tutorial is probably written this way because it states an assumed minimum of 8GB and allocates 5GB to the persistence area. Since different people/resources will calculate the size of a GB in different ways using 2.5GB + 5GB allows some slack in number conversion, etc.
Lastly you never actually told us what the issue is that you've run into (was it just a matter of figuring out what the tutorial is actually trying to say?). Why do you feel your primary partition needs to be more than 2.5GB?