It sounds to me like it may be a resolution setting that is set incorrectly, if you just change the page size in the property bar manually it won't export correctly because the default dpi setting is 300dpi.
I'm not sure if your problem is simply that you're going to FILE > EXPORT (as jpeg) or if you're using FILE>EXPORT FOR WEB but you may want to try my example below to see what the working resolution of your document is and if its 300dpi to switch it to 96dpi (the maximum screen resolution displayed by monitors on windows machines is 96dpi) this should fix your problem if i understood it correctly.
Once you've set the page size go to TOOLS > OPTIONS
You can set your dimensions in the property bar or in the below settings. Now when you go to export the file it will export it at the pixel dimension you've specified and at 96dpi.
If this doesn't work maybe you can be a little more thorough explaining the problem so that you get better suggestions...Good Luck!