I'm trying to install a saddle valve for an icemaker/refrigerator. It says the line should be 1/2 inch that the saddle valve goes on. Can it be 3/8 inch instead?

No, in case of 3/8" pipe, saddle valve will not grab well and will leak. Use double angle stop.

I'm trying to install a saddle valve for an icemaker/refrigerator. It says the line should be 1/2 inch that the saddle valve goes on. Can it be 3/8 inch instead?

I'm trying to install a saddle valve for an icemaker/refrigerator. It says the line should be 1/2 inch that the saddle valve goes on. Can it be 3/8 inch instead?
Milo gave you good advice. We don't recomebd using saddle valves. Change out your angle stop for a double angle stop, (see image).
Simply remove the hot water angle stop leaving the old compression nut and ferrule. Install the new angle stop and reduce down to install the ice maker line. Good luck, Tom