DPI is irrelevant in this context. It’s relevant to print production, but not when the image remains digital. You can test this by changing the DPI in Photoshop (or equivalent)—as long as you do not resample (which will reduce or increase the amount of data in the file, and it will look smaller, or bigger, on screen) the image will be unchanged and will still contain the same number of pixels (however, it will print at a different size). So you can ignore DPI. Just look at “pixel dimensions”. This shows how many pixels there are in the image. As for how many pixels to use per side, this depends on the size (and shape) of the surface you are texturing, and the quality (resolution) you wish it have. High resolution body maps in DAZ will be around 4000 x 4000 each, but the eye surface map will, as you would expect, be much smaller. Personally, I tend to look at the size of the maps used on a similar professionally produced model of the type and quality I want to go for, and follow that.