How many WS2812?
The datasheet for that part has no current consumption vs brightness info, so I'd start by measuring current consumption at my desired brightness. Multiply this by the number of LEDs, and that's the drain current the MOSFET needs to handle. A BSS806 might switch fast enough, and has Rds(on) on the order of tens of \$m\Omega\$ so can easily handle 1.5A - as long as you switch it fast enough.
By the way, if the WS2812's power on in an off state, then switching speed may not matter.