'page size' is a fixed value and can be determined by querying @@maxpagesize. You don't 'calculate' any page sizes.
To define thresholds (via sp_addthreshold) you need to know the number of pages (not necessarily the size of the pages).
Thresholds are defined in terms of number of pages.
sysusages.size is the number of pages allocated on a disk fragment.
If you're defining thresholds as a percentage of the segment (eg, thresholds at 10%, 20% and 30% free), then you just need sum(sysusages.size) for the segment in question (this should equal the total_pages value you see in sp_helpsegment), multiply by your %, and you've got the number of pages needed to define the threshold. Net result is all calculations are done in terms of number of pages, ie, no need to convert to/from MB. Since the calculation to determine thresholds is based solely on number of pages ... the page size never comes into question ... so you can use the same threshold calculations regardless of page size (@@maxpagesize).
Now, if you still want to work in terms of (mega)bytes, there's nothing to keep you from multiplying your number of pages by @@maxpagesize to get bytes (obviously divide by 1024*1024 to get MBs). Obviously you'll then need to divide by @@maxpagesize to get back to a number of pages to supply to sp_addthreshold.