I could be wrong, but here's what I think is happening.
This plugin isn't generating thumbnails, it's using the thumbnails that wordpress creates when you upload an image (I discovered this because I changed the setting in this plugin to 300x300 and my images came out all pixelated-- the reason being that it was scaling a 150x150 image-- the default thumbnail size that wordpress generated an image for when I uploaded my images-- up to 300x300).
What you need to do if you want your thumbnails a specific size is in WordPress, go to "settings > media" and change the thumbnail size to be the proportions / size you want. Note that this will only affect images that you upload after you change that setting (unless there's another plug-in that can automatically go through your images and regenerate thumbnails.)
The "Thumbnails for Excerpts" plug-in can scale down that thumbnail to whatever size you want, but it'll be the same proportions. If it forced your 150x150 thumbnail to 100x50, it'd be all distorted.
Hope this helps.