Wahoo - 10 Megapixels are more than enough... I have made 20 x 30 prints that are absolutely beautiful from a 6 MP Digital SLR and 11 x 14 from a 2 MP Digital SLR.
Are you shooting your images in RAW or JPEG? If JPEG, what size do you have it set to? Usually you can set it to take many different sizes depending on how small you want the file size. I always shoot in the largest size possible, flash memory is cheap nowadays so I don't worry about using small file sizes to save space. I always shoot in RAW mode as well and do all post processing in Adobe Lightroom and Adobe Photoshop CS3, much better control over the end result as opposed to letting the camera do the processing.
You can interpolate a small file size up to a larger size, but you can only do so much before you start to see different artifacts in the image.
From the sounds of it you need to shoot your photos at the largest size possible for your camera and also make sure that the compression setting you are using is for best quality and not best file size.
PM me if you need any more personal assistance.
Kevin.