forget size in inches. The projector will have a size in pixels - most often 1024x768, but check what yours is.
Resize the images to this pixel resolution.
You will probably need to set the image colour space to SRGB.
for the fastest projection speed save as best quality JPG.
You need to test what the images look like. You may need to apply some sharpening, but you need to do this carefully for best results.