A EPSON GT-15000 is an A3 scanner (297 x 420mm or 11.7 x 16.5 inches).
Using macOS scanner driver, it scans ok from HighSierra 10.13.6, the highest macOS this iMac can run. In the scan dialog are presets for the paper format, but only A4 and US LTR, not A3.
The only way to get an A3 scan is by manually setting the paper size to 297 x 420mm, this works but is fiddly. EPSON’s own app “EPSON Scan 2” can’t recognise this old scanner from 2003.
Vuescan can do it, but as it’s only a paper format question, I try to avoid a paid-for driver.
Where does the Apple scanner driver read the available paper formats from, there must be a hidden file. If I can find it, I hope I can add the missing paper format.
I currently have a Brother printer/scanner, and I see something that looks like a model to paper size mapping in /Library/Image Capture/Devices/Brother Scanner.app/Contents/Resources/DeviceSpec.plist, on my 10.13 Mac. I’ve only had a quick look at it though and haven’t checked it out in any detail. Oddly, I can’t quickly find my printer model in there.
I have a more recent Mac, running macOS 12.4, and my Brother scanner did not appear in Image Capture.app there until I downloaded the relevant scanner driver from the Brother website. So I believe the folder I mention above was created when I installed that driver. The file containing the driver was named Brother_ScannerDrivers_ICA_1_12_1.dmg - note “ICA”, which apparently stands for Image Capture Architecture.
I have plugged my 10.13 Mac into an Epson printer/scanner in the past, and on that machine, I also have the folder /Library/Image Capture/Devices/EPSON Scanner.app/Contents/Resources/. (I remember the Mac automatically downloading some drivers when I plugged it in to this printer, so perhaps this folder was created at that point - it doesn’t exist on my more recent Mac.) There are a few .plist files there also, but they don’t seem to have the same structure as the Brother ones. I do see a list of DocumentType page sizes in ScannerCapability.plist, but I don’t immediately see how that is mapped to (printer/scanner device) models.
So - not an answer, but perhaps this gives a promising place to look?
It does indeed give me some pointers, many thanks @ashley.
In the meantime I received an answer from Epson support, they are are not very interested in helping with what is admittedly quite an old scanner.