A file size for a photoshop file depends on many things but an average (finished file) might be in the area of 50mb.
The dimensions for a file should be at maximum print size and no larger. That might be in the area of 12"wide x 16" tall. or for larger pallets, 14-16 wide.
The max resolution (pixels per inch) should be no higher than 300 ppi.(pixels per inch). This is great for separations and printing for other products that might need high rez files like off set printing (posters, calenders, Magazine ads, etc. For screen printing, the rezolution (at actual perint size) shoul dbe at the least, (2 times your line screen halftone you will use for print. e.g. 55line screen will need at least a 110 resolution file. This used to be standard (due to trying to keep file size down) but these days, we have plenty of ram and room so it's very common to keep it at 300 resolution.
To keep your file size down, merge/combine layers that are not touching so that you can still use the elements as a selection /mask later. Keep the number of layers down. The less you have, the smaller the file size. 10-20 is average, more than that is high. No need to have 300 layers.
A final note, Create all art in RGB and keep it as the master file. If you need CMYK, let the printer convert that to CMYK.