The tabs are 14-15mm, they list all the dimensions on the product page as well
https://www.amainhobbies.com/ecopow...digital-sub-micro-servo-trx4-ecp-640t/p859667
As for notching the bumper to make it fit, I'm using a 3D printed bumper so I cant say whether the stock bumper will clear with some notching or not.
EDIT: I just mocked up the stock bumper mount and it clears though the bumper will need some trimming, but I'm fairly sure it can be made to work. It might not require a Dremel to notch, All I used was a utility knife to notch mine.
Size of the screws...thats a good question. I've tried to figure that out and I still have no Idea, they dont match up to any of my spare screws.
One idea might be to steal the long screws that connect the body hinge to the bumper mount, these can be used to mount the servo. Then if you can find a tiny wire maybe from a twist tie, run it through the hinge and secure it, Or maybe the short screws that fasten the stock servo will work here temporarily.
Oh and one other thing to keep in mind is the servo horn provided with the 640T has the holes at the wrong distance compared to stock and the holes are too big unless you can round up a nut or something. I drilled a new hole (1mm-ish) sort of offset and in between the other holes to get it in the same spot as stock.