Assuming the size has no limits, and is dynamic therefore it has no set size, how many marines do you think are in it at one time on average?

How many marines would a chapter lend out at one time? if every chapter, given there are 1000 chapters, loaned 1 marine, this would make the Deathwatch standard chapter size.

Now imagine there were an average 5 from every chapter that would make the deathwatch rather huge. maybe being able to fragment itself.

Now if there were an average of ten (which would might possible as i assume there is more than 1000 chapters nowadays allowing for a higher and lower amount of marines to make the average.

Imagine the deathwatch fielding all the marines it has at once in one place.
We would be talking a Great crusade legion sized force. and thats not counting all the ancillery forces, inducted guard, sisters, inquisitors and the like.

Sounds like a party.