Try the attached file. Also converts .dds to .bmp.
Make sure that you want ALL the bitmaps in the folder to be 1024x1024, because this will increase the size of anything smaller than it!
Edit to change the size this batch creates, but it will resize ALL files in the folder to the same size.
This DOES NOT WORK for DXT5 files - imagetool chokes on them.
I wrote the .bat file - imagetool by ACES, nconvert can be found here -->
http://www.xnview.com/en/nconvert/
Nconvert has an older brother - Xnconvert - with a GUI. I really like Xnconvert - it is a genius program that produces excellent results at a truly stunning speed. However, this batch file I've attached here is much easier to use and does not require installation to a specific location or any mucking around in GUIs, so it's faster for my purposes.