This doesn't change the actual dimensions of your graphic or project, it just makes the file size smaller.

Click on "File / Export / JPEG Optimizer". 
Where it says "Set Compression value to...." is where you do this. 
1 is the lowest, basically no compression, and the higher the number, the more compressed it is.
Then click on OK and save to your folder of choice.

For an example of how much to compress, I once scanned something and it was around 750 KB.
I compressed it at 30 and it wound up being around 175 KB, and it didn't seem to lose any quality.
Just save the original in PSP format so you can use that for your own printing, and in case you need to adjust the compression value again.

Another way to compress is to click on "File / Save As".
At the box that pops up, click on the OPTIONS button near the bottom.
You can slide the bar in the compression factor here and can see the numbers change to what you want.

~Summer °Ü°