OK so after a bit of playing around in Interface Builder, I've noticed that I can switch off the "Use Auto Layout" feature in the File Inspector. Now when I switch to the Sizing Inspector I have an easy to use "Auto Sizing" feature that when I hover over the preview, shows me exactly how the NSTextField will size when re-sizing the parent document.
Also once I've switched off "Use Auto Layout", I can use the mouse pointer to adjust the NSTextField height and width in the Interface Builder.
I guess the "Use Auto Layout" is constraining the height of the NSTextField to the appropriate height it thinks it should be but I can't tell where it get's this value from.