I tried out 3 mains of Special Power Up with Bubble Blower, and I think it was about a 33% size and explosion increase? Atleast I remember it being fairly significant.
I personally would only run Special Power Up on Splat Zones. It makes your bubbles more likely to instantly take over the zone. I have had many matches where my bubbles turned things around. For everything else, eeeeeh. Bubbles are still useful, but the size doesn't mean much in other modes. Most players either steer clear or get stupidly close to your bubbles. For Splat Zones, I just run 3 subs of Special Power Up. It is usually enough to instantly take over the zone, or one bubble can instantly take over the mini zones.
I recall Bubble Blower being considered the worst special, but I think it is really helpful. I mentioned its value in Splat Zones, but it is great for going through choke points or making a push. Release your bubbles, swim under them, and if the enemies try to get you, suction bomb the bubbles and they die. Players also like blowing their cover just to shoot at your bubbles.
Now bubbles need something where Ink Storm doesn't do damage to you if you're hiding under a bubble.