That fixSize couldn't work without you implement it in wall.reset({});. Put it like this:
wall.reset({
fixSize:1;
// other options...
});
According to Freewall's options documentation:
fixSize : boolean
Default: null
value is null or not set let blocks can adjust and resize to fill gap
value is 0 or false let blocks can adjust size but can't resize to fill gap.
value is 1 or true let blocks keep the size as init, can't resize to fill gap.
Can override the fixSize option by set data-fixSize attribute in the block.
There's two fixSize: one is a method, while other is an option. The first one wouldn't work if the former isn't defined.