How great a reduction do you have to make? (if great is the correct word here).
I use a different editing programme but the principle is the same.
If making a substantial downward resize set your image quality settings to give best quality. Type in your required image size and resolution. Select a suitable Jpeg compression setting. As Eeoyre said.
But a couple of things to watch. For best quality, never use any of the auto 'size for web' etc settings; doing it manually will take a bit longer but gives better quality. For best quality images (eg for my P base site) I use 100% (best quality) Jpeg setting. Or 90% for photos going to the WAB
Gallery. I wouldn't recommend going below 70%.
I usually set the resolution to 96 ppi which is the average European monitor standard, although Americans prefer 72 ppi.
After a substantial resize your photo will benefit from a slight sharpen, preferably with Unsharp Mask; but don't overdo this.
One other point to watch is to check that your new sizes conform to the required minimum size in both directions. I normally use 800 pixels wide for P base but a 3 x 2 ratio image size for the WAB best quality
Gallery will be rejected because your height is now too low. For this, I use a width of 1024 pixels (average screen size) which gives a suitable height. But this is just what I use and won't be correct for all circumstances.
Hope this makes sense. In reality it is a lot easier than it sounds.