Maybe he just know it? It was discussed in maemo lists (or on irc?). Someone asked why they added exactly 256MB of flash (too smal in his opinion) or maybe why they added it at all instead of some other solution (don't remember, maybe small flash + boot from bigger internal mmc) and someone from nokia responded that it comes in one chip with the CPU so they just used it as is. Also something along the lines that if one has bad blocks in flash it is bad because the flash chip cannot be replaced alone but together with CPU.