To compensate for the small size of PSP brushes (as against Photoshop ones) put each brush stroke/stamp on a new layer if you want it larger and drag the deform box until it is the required size.
Also, if you have an image you wish to make into a brush, open it, convert to black/white, then select, go to File>Export>Custom Brush, da, da you've now made a brush.
You can add noise or other filters, erase parts of it out etc. after you have converted to B/W but before you select. The maximum size is 500 pixels so it is easier to open a new document and fill with the image you wish to make into a brush.
HTH, if you need any further help, holler!
Jules
