If you use the anchors, you can have the size and locations of the fields adjust according to the size of the monitor.
You probable already checked this, but occasionally I find users that are liking at layouts at view values >100%, making if look like the interface doesn't fit.
I've found that two users are unable to view the entire GUI on their monitors. The effect is as if they're operating at a low resolution (i.e., the GUI has ballooned in size and they have to scroll up and down to access its controls), but I checked their monitors, and they're set at 1440 x 980. The monitors are older Viewsonic models (that coincidently show a really horrible and washed out image, besides).
If the issue is on Windows machines, you may want to check the following:
Edit > Preferences
Ensure that "Enlarge window contents to improve readability" is disabled
Also, on Windows again (7 anyway) if you right click the desktop and choose Personalize, then Display in the lower left, you are able to change the scaling used from 100% to 125/150% which may be the issue.
Im unsure if there are similar settings on the mac, being primarily a windows user myself.
I'd like to thank you all for taking the time to response... helpful thoughts, all...
It's thankfully not a matter of the users zooming the layouts; I checked that myself first thing. And to make sure it doesn't become an issue in the future, I added a sniffer function on the "Open" script which first resize's the window to fit the layout, then "sniffs" the user's OS; if it comes up Windows, it'll unzoom to 75% and lock the magnification so that the user can't change it. I had to go with this 75% for the Window's users because I can't for the life of me determine why I can't see the entire layout notwithstanding the fact that their resolution setting match those of the Apple users.
I only wish that Filemaker offered 10% increments with respect to the "Adjust Window" script command (as opposed to the 25% increments), but having the option at all is a sheer blessing, as I otherwise would've had to set up a low res and hi res version of each layout.
I'll check Steve's preference setting mentioned above, and with respect to future solutions, I'll look into doing dual versions layouts for hi and lo res using anchors as Taylor prescribed.