I just bought the plans and at the end when it gives you a list of all the parts needed it says a peice of tempered glass. What does this mean and how big should it be.

I don't know the exact dimensions off the top of my head, but tempered glass is glass which has been hardened and strenthened by means of extreme heat. You can order it at most hardware stores.

The tempered glass should be the size of your enclosure minus 1/2" for airflow under the glass. Read the section in the guide for cooling again to try to make sense.