Changing image sizes

Yes, it is.

When setting up any layout, look at the right-hand column of the field you are using for the uploaded image (the ‘Image File’ field in the sample layout). There you can set the maximum dimensions, maximum file size, image format and other things.

As the author of the PAGB (UK clubs) projection standards, can I correct any misunderstanding.
There is, and never has been, any UK club standard for projection size. The standard is for the event organiser to state the projection size in use, whatever that may be.
The most prevalent club standard at the moment is probably SXGA+ (1400×1050), but that is an evolution from XGA (1024×768). The PAGB, many federations and some clubs have moved to UXGA (1600×1200). I believe there are also clubs using widescreen formats rather than 4:3. It is absolutely their own choice, as clubs are all autonomous.
As Josie says, your maximum image size can and should be set in your Layout.

“SXGA+ (1400×1050)”
What is the size file setting you are using?

I use an EWZ upload file size maximum of 2MB for UXGA. Experimentally, files from multiple photographers for SXGA+ projection average about 750KB and those for UXGA average about 1MB. There are situations where image files can be much larger, but it is not desirable for an event organiser to allow enormous files into the system. The Photographic Alliance of Great Britain has published guidance on managing image file sizes, which you can see on their website.

Thanks for the input.