What image size/resolution should I use for a sublimated jersey?

I'm trying to get a sublimated ultimate jersey made by Spin Ultimate. I've created an image for it using the illustrator template they gave me, but the nebula/space image that I'm using for the jersey I created in photoshop. The guy from Spin that is collaborating with me asked for the individual psd file and specified that the sub printers prefer at least 300 DPI resolution...

I don't ever do much printing so I'm confused as to how to size my photoshop image for placement in the Ai template (which is about 40 inches by 32 inches). He then wants me to email the psd file, which is huge!

I apologize in advance for my ignorance and incompetence.