Sharpen one size only?

Hi,

I found this script online and it looks excellent, thanks! I am probably going to add it into my functions.php file rather than using the plugin, so that I can modify it a bit.

How could I modify it so that it only sharpens particular sizes? For example, to sharpen the “thumbnail” size but not the rest. Or to sharpen all sizes except for the “large” size?

This plugin might be even better if it had an options page to let you choose which sizes to sharpen. However, it is good that you added the filters. Perhaps you could suggest filters for sharpening specific sizes?