The keypad I got was about $1.25 from China, its about 7.5cm (4.5im) long X 6.5cm (3.5in) wide.
The A+ fits within the white border of the inside box of this keypad.
I also got a 4X4 key pad which is wider than the first one but it has not arrived yet. It was 85¢ from Hong Kong.
Just have to figure out the matrix wiring before connecting it to the R-Pi. There are a couple of webpages which shows how to connect other keypads to the GPIO. The program itself is simple. The issue on all of them was the matrix wiring.
The first key pad is 7 wires for 4 rows X 3 columns The second keypad is 8 wires for 4 rows X 4 columns.