Crop and save (JPEG high quality, should be OK, if no further editing is needed).
If you really need to look at the file size, look at a critical part of your file (ie a part with high contrast) at 100 % and with priview enabled save as JPEG and use the quality parameter to lower the quality. As long as you do not see a degradation, you can go on...
No prob. You're welcome. I'm even not sure that you will need the 300 dpi. At that size, I suppose that 150-200 dpi is more then enough. Your service provider may answer that question.
Also: When you edit the file to correct the colour, stamp out imperfections, you should a) work non destructive (layers) and save as PSD or TIFF (with layers). You will send to your service provider a flattened JPEG file however.