Real size image, transparent background, 200dpi.
My opinion:
Set the dpi, depend on your design, and your printing machine. I mean if you working with many effect or many "smart object", and you just print it with digital printer, you don't need to use 300dpi.
For complex illustration, I usually using 40x60cm document size with 150 or 200dpi. I always keep all layers to prevent client revision, and usually my file size are around 700Mb-1Gb.
I can't work with 300dpi because I need to work efficient and faster, and if I'm using 300dpi, my pc will gonna blow
And to make a film I usually using 72 or 125 dpi machine, so if I working with 40x60cm 150dpi, I can get 80x120cm 72dpi. More than enough for screen printing right?
*Except for offset printing, I'll work with 300dpi.
Hope that helps (sorry for my grammar ;P)
When designing for DtG in Photoshop, I start with the size of the printer platten, 14x18 inches at 150dpi.
After the design is proofed (low res 72dpi .jpg with garment color background), I then crop to within a 1/4 of design, save out as a transparent .png.
Import to the rip and let it do its magic.
NOTE: Threre is NO visible difference between 300dpi vs 150dpi initial design file when it comes to the final print.
Later...
I agree with you, there's no different between 150 and 300dpi, because to print the screen printing film, we're using digital printing,
I never heard anyone who make a screen printing film with offset machine, so we don't need 300dpi
