Changing the amount of "preview" text

Was just wondering if it’s possible to change the length of the text excerpt for posts on the homepage, as WordPress has it’s own read more button/tag built in for it I’d prefer if the excerpts ended when I used this tag rather than when it hits a character limit as it does at the moment. (I’m using Small Thumbnails and display excerpts).

I know this may cause problems with some posts getting too long, but I use rather short sentences before the read more tag, so currently the excerpts are longer than I’d like them to be.

Is there any way to change this?