First off -- the game will save them in the size that your screen resolution is in game -- so you'll only get as bug as the resolution that you play the game at (or the size your monittor can display - if you increase the size in the game options - so say you normally play at 1280x800 but your monitor max is 1680x1050 then before launching to tke screenshots change your game res to the max your monitor supports to ge the largest screenie you can !)
2nd - convert your screenshots to .jpg -- .jpg compression will make a smaller filesize and some sites restrict the filesize they allow uploaded so using the .bmp format will result in the screen being downsized to fit within the allowed upload size !! (photobucket allows up to 1MB filesize IIRC and if saved in .jpg format I use about 756KB for a 3,264px × 2,448px image so they can be pretty large and not get cropped !) -- http://i212.photobucket.com/albums/cc189/JDFanning_bucket/ScreenShot4.jpg - and when I make the filesize larger than 1MB. using a higher quality image with less compression it downsizes the image but if I use a tighter compression to keep it under 1MB. the fullsize image is uploaded - so make sure you keep the .jpg filesize under 1 MB. and they should remain fullsize . (I doubled the screenshot size using Photoshop and then saved with both the highest quality .jpg which resulted in 3.7Mb filesize and again with medium quality compression for 907Kb. filesize -- the 3.7Mb. one was auto downsized to 1672x1045 while the 907Kb. with higher compression was left fullsize !)
3rd. when uploading be sure you are uploading a full size image and have selected the 1MB filesize (they have some presets for automatic cropping and if you choose those all images will get downsized) and then when posting a link use the direct link code