Always take your measurements from the back of the frame, making sure that you measure across the inside of the rebate. You then have two options - take the measurements to a glazier or have a go yourself. Glass cutting is easy once you have some practice, and the right tools.
2mm thick glass for picture framing - this will fit into most moulding rebates and it is not too heavy. Clear, glass is used most commonly for definition of the artwork. Use non-reflective glass in brightly lit areas where reflection is a problem. This glass may effect the clarity of your artwork, especially if the glass is a long way away from the surface of the artwork - for example on paper tole, triple mats and shadow box frames. For these jobs use plain glass.
The FrameCo Glass Cutter has a Tungsten-Carbide head mounted onto a solid aluminium base and will provide years of glass cutting. The head is vertical to the glass surface at all times and this ensures the cut is finer and more even each time. Attach to the MatMaster Rule and you have a glass cutting tool that is easy and safe to use. Less danger from broken or miscut glass.