The bookmarklet (My Account > Bookmarklet) is simply a snippet of javascript that opens a new window and pre-populates some information in the EE entry form for you. You can alter it by altering the javascript.
Credit for this tip goes to spacey310:
1. Right click on the link, and select “Properties”
2. Copy everything in the “Location” field into a text editor
3. At the very end the text is something like
'bmentry',''))
4. Replace that with
'bmentry','width=700,height=520,toolbar=0,status=1,location=0,
scrollbars=1,menubar=1,resizable=1'))
You can specify the size you want as well as the various window options.