I think Lattice has the smallest and cheapest FPGAs, like the ICE40LP640-SWG16TR50, which is only 1,4 mm x 1,5 mm, but has still 640 logic cells, more than enough for Manchester decoding. But might be difficult to route and to solder. For Xilinx you could use a XC9572XL-10VQG44C. Much bigger, 10 mm x 10 mm, but can be even hand soldered, and works with 3.3 V (the Lattice part needs 1.2 V). You should try first if your design fits in it.