I do not entirely follow the question. Given A=3 by 4 and B=4 by 3 I can see that the convolution is A**B = 6 by 6 but I do not understand this statement

"are circularly convolved using (6 x 6)-point 2-D DFTs (Discrete Fourier Transforms)"

do you mean convolving A and B but taking thier DFT multiplying and then doing an inverse DFT ? If you you do then the outputs are the same as A**B for all inputs A,B because of the convolution theorem.

"are circularly convolved using (6 x 6)-point 2-D DFTs (Discrete Fourier Transforms)"

do you mean convolving A and B but taking thier DFT multiplying and then doing an inverse DFT ? If you you do then the outputs are the same as A**B for all inputs A,B because of the convolution theorem.