There are no issues with the firmware fitting on the cards of most types. I think the minimum requirement is 32 Mb. As stated, just about any one will do. If I'm not mistaken though, the firmware is used to allow the use of SDHC cards in your camera. Pentax released several such updates about a year ago. I assume you've been using the camera so copy the BIN file to the card you've been using, make sure you have a set of Fully Charged Batteries in the camera, and have at it.
After re-reading your post...
Download the file to your hard drive and execute the EXE. It will automatically unpack and result in 2 files. A BIN file (firmware) and a Text file that contains the instructions. Use your 2Gb card for the update.