40 to 80 watts I'd say.
Size is not the only factor.
Less efficient cells = larger panel.
More efficient cells = smaller panel
My 75 watt panel is just about the same size as you describe.
Copied from here:
There are three types of Solar Cells on the market.
Amorphous or Dual Junction Solar Glass - Cheapest but only 7% energy efficiency and 5 years service life. It is big and heavy. If the seller didn't specify, it is the Amorphorus. It is black solid sheet in appearance. It is about half the price per watt compare to Mono-Crystalline. Most of the solar panels on Ebay are of this type.
Poly-Crystalline - 25 plus years service life under direct sunlight and 13% efficiency. It is blue color circular or semi-circular silicon wafer form in appearance.
Mono-Crystalline - 25 plus years service life and up to 20% efficiency. This is the same type use in solar car racing. This type of solar cells is now in worldwide shortage because only a few companies can manufacture this type solar cell. The raw material which is a special kind of silicon sand ore is also in great shortage. It is in black color circular or semi-circular silicon wafer form in appearance