They should be sending 10-24 studs with the letters. If so, you want a 3/16" bit. There's no reason to go any larger than that. As mentioned, in masonry, the holes are rarely drilled without some wandering, which is a good thing because it places tension on the studs. I don't think you'll need to tape the letters in place or anything.
3/16-inch masonry bit is the standard for a 10-24 threaded stud. A number 10 stud is actually the same diameter as a 3/16 bit, but a masonry bit will always make the hole slightly oversize through the hammering action of the drill.
A carefully drilled pattern will circumvent most fit problems. In the event you do get a bad hole, Gorilla tape sticks well even in cold weather. I try to use a skinny strip of it to minimize glue residue. Another option is this: If a dry fit reveals a problem letter, perhaps one that tries to lean away at the top, instead of taping it to the wall while the adhesive sets up, wrap some tape around the loose-fitting stud before you coat it with glue. Not to cover the entire stud with tape, but just enough to increase its diameter a little at one spot in its length for a tighter fit in a sloppy hole. This can solve the problem sometimes without the need for a return trip to remove tape from the face of the letter.
I have installed many studded letters to interior walls with no silicone at all, just inserting the studs into the holes either with or without a wrap of masking tape at the base of the stud. In drywall, the holes can even be made with an awl, tapping it in, rather than drilling, eliminating the need for a vacuum cleanup. Of course, the presence of a wood or metal wall stud may require a drilled hole.
Though 3/16-inch is the go-to bit size for exterior letters, I also kept a single 1/4-inch masonry bit on hand in case I needed to oversize an occasional hole to avoid putting a letter in a bind. Gemini formed letters do not crack easily because of the type of plastic they use, but a flat-cut acrylic letter can easily crack in cold weather.
For new installers: it's usually a waste of time to squirt silicone into the holes. Half the time the wad simply gets pushed to the back of the hole by the stud without engulfing the stud. Rather, always coat the stud itself by inserting it into the nozzle of the silicone tube. You can control the amount of silicone deposited on the stud by squeezing the trigger as you withdraw it. The hole on the nozzle doesn't need to be large, just big enough to insert the stud all the way to the glue block. This method also coats the stud evenly, creating less mess.
Brad in Kansas City
I had a customer request that once. I advised against it, but he was convinced if I didn't use it, kids would rip the letters out. Well, fast forward a couple years and a new guy moved in. They ended up replacing the whole block monument (small like 3x6) because they couldn't get any of the studs out and didn't want to just grind smooth.
