It'll depend on the sidebar widths that the blogger is using. Mine for example is 240 pixels wide though I use no image over 200 pixels wide. Generally I keep the images down to maximum 300 pixels height as well. You can go longer but if its for affiliates, I'd recommend about 160 pixel width and max height of 300 pixels.

That should fit most sidebars even if they go with a 3 column blog.

hth
Ian