5/16 pex to 1/4 to 1/2 for ice maker outlet box
Adapting the size will work fine as long as there is enough flow to fill. some units are on timers and if the flow is low this reduces the quantity of ice made with each batch. I have found fridges connected with saddle valves and a tiny orifice of only 1/16" was sufficient until the tiny orifice in the piercing screw got plugged. I don't like saddle valves because in my experiance they always leak. I usually add a double outlet valve or a t and a shutoff where I tap the water line in case the line to the fridge gets kinked. My wife is OCD with cleaning and the fridge regularly gets pulled out for cleaning and has pulled the line loose in the past so a shutoff at the point of connection is mandatory at my house and a good idea every place else.