If you uncheck the Resize to Fit box, then it won't do any resampling. And your pixel dimensions will remain unchanged. As for adjusting file size via jpeg compression--which if I understand you correctly, is what you are after--look under the File Settings section, and with format Jpeg selected, a Quality slider will be evident. Adjusting that slider changes file size and image quality, with lower values yielding smaller file sizes but lower quality. A setting of about 55 is considered moderate quality, and of about 85 is high quality. Experiment to see which setting serves you best.