Doors and Windows - making window frames

I want to make some fixed window frames for a sunroom. This would be a second window pane for winter insulation, but it would be on the inside so weathering is not a big problem. I bought some 1/8 inch thick glass, and was going to make some pine frames, but I cannot find information on how to make them. I heard that the pane has to "float" so it does not break with expansion and contraction of the frame. Is there a rule of thumb on how much extra space to leave in the dado?

The glass should be at least 1/8" less than the frame opening in both width & height. If you are holding the glass in with moldings, apply a light bead of caulk to set the glass into the frame (to prevent rattle) and then nail the molding so that it just sits against the glass.