Hello JadeKiaraIf the borders are x cm wide, then you have to subtract 2x from the overall dimensions (120 by 80) to get the dimensions of the central panel. In other words, the panel will measure (120 - 2x) cm by (80 - 2x) cm. The central panel has possible dimensions, then, between 120 cm by 80 cm, (if x = 0) and 40 cm by 0 cm (if x = 40), with the length of the panel always being 40 cm more than its width.

Unless you have any more information, that's all you can say.

Grandad