It is not that much of a problem. In fact, if you CTRL+Leftklick on the outer edges and look in the lower righthand side of the SW window, the length of the lines will show up there. Do two measurements and calculate an average, if you need it to be more accurate.
If you want a proper fold-out of your plywood sheet, try making it as a Sheetmetal part instead. It looks like it would unfold nicely, there will be a button in the toolbar for that.
In addition to what Bengt suggested, if you made the part as a Thin Extrusion with the thickness growing to both sides of the sketched profile, then you can just measure the total length of the profile. If the profile is lines and arcs, then you can dimension the length along each arc (ctrl-click on the two end points and the arc segment) and line, add them all together in an equation, and then link that equation output to a property you can show on your drawing. There is usually more than one way to skin a SolidWorks cat. (Of course, using the Sheetmetal tools is a cleaner and easier way to get a nice drawing.)