I am not sure what metric size bolts I need?

The best way is to take a sample of your bolt or nut to a local harware store. They most likely will not have these sizes in stock, but they should be able to tell you exactly what size it is by comparing it to a template they have or looking it up in a catalog.

The other way is to get a metric tape measure or a more precise tool and measure the thickness of the bolt and its lenght.

The most standard sizes are M6, M8, M10 which means 6mm, 8mm, or 10mm diameter (this is rounded to compensate the cutting of the tread). Then its the lenght of the bolt, which also measures in mm as 25mm, for example, which woulod be close to 1 inch long. Thirdly, it is very important to get the correct thread size, which is the distance between the actual cuts of the thread. They, too are measured in mm, like 1.25mm being the most common size.