For the record, this unit has a compressor - it is not Peltier cooled. It does have a small block at the bottom where the compressor fits into the back. My 30L fermentor fits in front of the block with a little room to spare between it and the glass. It doesn't fit on top of the block (too high), but a smaller fermentor might.
The block is 14cm back to front. The floor in front of the block is 31cm back to front. The inside roof has the control unit at the front-middle, which might kill a few cm depending on what is going in there. Inside height from block to top (bottom of control unit) is ~48cm. From floor to top is ~69cm. So the biggest cylindrical fermentor you could fit in there without modifying the insides would be around 31cm diameter, 69cm high.
If anyone wants photos or any other specs let me know.