I'd upload them directly to a printing place rather than go through the trouble of burning a CD. Burning CDs is often a painful exercise in failure... my opinion... :-)
First, the answer to your question.
I'll presume you're printing 4x6 or 5x7 or something? For printing at that size, you really don't need a huge version of the photo. If you're printing an 8x10 or an 11x17, yeah, you definitely want all the bits - the highest resolution you can get.
Here's what you do. 1. Select the photos you want to print and put them in an album (a blue-book album). 2. Click on the blue-book album you've created (in Picasa's folder list, left side of the Library View) to select it. 3. Do a Create -> "Create Gift CD" menu choice. 4. Verify that "Photo Size" is set to "Original Size". 5. Click the "Burn Disc" button.
Personally, once you've created the album (to nicely select the photos), rather than burn the Gift CD, I'd upload them directly to a printing place... In Picasa, you can click on the album and then click the "Shop" button and, if things are working nicely, order prints of those photos at your choice of many on-line printing places (e.g., Shutterfly).