I've created a project which displays absolutely fine on my own screens within Captivate Prime but for a colleague it displays too large and she has to scroll down the screen to see the bottom of the slides - is there some setting that I can switch on that means the slides display correctly according to the size of the learner's screen, including on the mobile app?
Are you talking about a responsive or a non-responsive project? If it is a responsive project, has it been created with Fluid Boxes or with Breakpoint views? Which exact version of Captivate do you use: full version number can be found under Help, About Captivate.
Check your own Display % setting in Windows to see whether it is 100% or greater, then check with your other users that are having issues to see if their display % is greater than yours. It could be that their display in the Windows OS is making everything too large and that is making scrolling necessary.
If it turns out that the default Display % setting for most users is say 125% (quite common) but you have set your project size to fill the available screen space on your own system which is set to 100%, then I think you are going to need to resize your project down to something that everyone can see without scrolling. This is one of those issues that you need to check for at the beginning of a project before you go too far in building modules.
If it turns out that the Windows Display % is not the issue, then the other likely candidate is the web browser Zoom % settings. That can also make things look too big.