If the Poser render is too small and you enlarge it greatly, it's not going to look good. The solution is to render to a new window at a larger image size. Images are so many pixels wide by so many pixels tall. Making them larger doesn't increase the information in those pixels. A 300x300 pixel image enlarger to 1200x1200 is going to look like garbage in Photoshop because Photoshop has to fill in all the missing pixels with it's best guess. It's not more detail, just bigger pixels with some guessed shading. A 1200x1200 render out of Poser will have 1200 times 1200 pixels of calculated information.