You can't make the pixel size any larger ("pixels is pixels"), so instead, what you'll want to do is write a function that will color the pixel of choice and the pixels around it (to make, say, a 3x3 pixel "block"). Think of the square as a grid of pixels, and it's easy to figure out the coordinates of the surrounding pixels.
Warning: Your function will need to deal with boundary conditions (e.g., what happens when you try to color all of the pixels that surround the pixel at 0,0?).
"One word of warning: if you meet a bunch of Perl programmers on the bus or something, don't look them in the eye. They've been known to try to convert the young into Perl monks." - Frank Willison