Reading the links Mac128 provided, 128K SIMMS do not exist. And if you mod a 256K SIMM you will get 1/4 the capacity or 64K.

So assuming I can MOD a 256K SIMM to 64K, here are the possible capacities for a Mac Plus:

128k

256k

512k

640k

1024k

2048k

2176k

2560k

4096k

So a 256K Mac Plus is possible in theory (four 64K SIMMS) as is a 128K (two 64K SIMMS).

Now I have something to do with my box of 256K SIMMS (if I can find them!)