How to measure surface area of plated bacteria?
You can count the individual colonies on the plate, with this number being the number of "Colony Forming Units" (CFU), as theoretically, only a single cell is needed to spawn a colony.

To do this though, you will need to do a serial dilution of your liquid cultures before plating, so you would have one plate of 10$^0$ dilution, then 10$^1$, 10$^2$, and so on, generally a ten-fold dilution is performed.

Hopefully this will yield at least one plate with a countable number of colonies, that is - not too many, and not too few - usually 30-300.

cfu/ml = (no. of colonies x dilution factor) / volume of culture plated

This usually yields a very large number, so it is common to then take the log of this number, and report that as your result.

And then you will be able to compare your results between probiotics. It's a very good idea to do replicates of each dilution!