[OUL]

Hm i was had similar problem before. Well i was this situation:

pawn Код:

fnc(playerid,cells,...)
{
    new
        string[ sizeof (cells) ];
    format(string, sizeof string,.....
    return playerid;
}

So my advice would be to try with

[sizeof(cell)]

In your case ( if I understood your problem good )

pawn Код:

new
   MAX_STRING = sizeof(128);
if(sscanf(params,"s["MAX_STRING"]"))