There is probably a minimum size below which the spreading center is too close to the subduction zone...the "slab: is too hot and buoyant to subduct. Thus, as the Farallon plate broke up about 30 million years ago, the microplates subducted faster than the half spreading rate.....and the ridges approached the trench...then, subduction stopped and spreading stopped.....and the former descendents of the Farallon plate became part of the Pacific plate...the fossil ridges are sitting out there today (for example, Davidson seamount):
refs...Nicholson et al, 1994, Lonsdale, 1991, ...others

I think the plate were about 200 x100 km when they were captured...but would have to look at maps..
Chris