I would not do that. Userspace Apps have way to many ways to screw up memory managment already. Allowing them to

    buf=malloc(128);
    free(buf, 256);

seems dangerous, if free can't check the size.

But kernels sometimes do just that ( free(ptr, size) ), for performance reasons and because "kernel writers know what they are doing".

That's a pretty nasty requirement on top of an allocator protocol; I wouldn't want to use it, or debug it for that matter, yikes.

And you don't need to store the size. The slab allocator doesn't store any information except how far into the current slab it's already dished out memory.