The extent to which you can safely enlarge your image (without resampling) depends on its format:
The quality of most ClipArt.com photos will hold up until they are enlarged to about 350% of their size. More than that may cause the image to look pixelated (blocky or jagged).
You should not enlarge a web GIF to more than 150% of its size, especially if you are going to print it. If you do, pixelization will occur.
Clipart -- JPEG and GIF
The quality of a ClipArt.com JPEG or non-web GIF will hold up until it is enlarged to about 350% of its size. More than that may cause the image to appear pixelated.
Clipart -- EPS and WMF
EPS and WMF images -- even web screen images -- will output high quality to any device no matter how much you enlarge them. If you wish, you can use a high-quality paint program like Photoshop, PHOTO-PAINT, or Paint Shop Pro to convert these to a raster format (like JPEG, GIF, or TIFF) after you have scaled them to the desired size.