What you're after is a range of eyepieces. The 25mm I suggested is the one that will give you the best "blend" of magnification and surface brightness. It's twice the focal ratio of your scope.

For wider field views, you can go up to seven times your focal ratio. However, that's a bit extreme in this case. Your scope only has a 1.25" focuser which most beginning scopes have. When you start shopping for longer eyepieces, you run into the fact that many require a 2" barrel in your focuser.

I recommend getting a 40mm eyepiece for your longest eyepiece. Here's a pretty decent Plossl eyepiece with plenty of eye relief that will fit in your focuser...

Stick with your 20mm eyepiece for a while, but if you decide to get a better quality 25mm eyepiece in the future, here's one that is 26mm (just fine)...

Clear Skies