Yes, you'll have to make sure you place a FG grid on the map that represents 5 feet if you want all of the FG measurements and token sizing to work correctly.
Even with a map that has a 5' grid on it, there can be times when you can't line up a FG grid accurately - due to the map not being exactly the same grid size across the whole area, bad scan/sizing, etc.. For some users it's OK to get "good enough" and nudge the grid up/down/left/right as actions changes across the map to get a better representation. It all depends on the map and what you're prepared to live with.
Some users will do pre-processing to get their map more FG ready - there's the map alignment tool already mentioned, but as you're on a Mac you probably can't use that without messing around with Wine or other windows emulation. Here's a video that gives an idea on how to better prepare maps (including sizing) using Paint Shop Pro (but the same principles can be applied to GIMP and PhotoShop):
https://www.fantasygrounds.com/filel...rescale-1a.wmv Of course, resizing can have it's own issue - introducing artifacts, losing some details, blurring, etc. - so it may take you a few attempts to work out a process that works well for you.