Ferrule size for a road brake cable housing slot? I have bought some generic cable housing without paying attention to the specification (https://www.bricklanebikes.co.uk/blb-brake-cable-outer-housing-black). It comes with a set of ferrules and I didn't know those have a size as well (mines are 5mm according to the link above). The ferrule fits very well into my lever (a bar short-pull straight bar lever), but it doesn't fit on the brake housing slot (see picture below). The brake is a Shimano 105 5800, but I can't find anywhere a mention of the size of the housing slot. (I tried to look even in the dealer's manual.)  

What's the standard size for the cable housing slot of modern road caliper brakes?

If I go with a cable housing (and ferrules) with smaller diameter, am I going to have issue on the lever side? (It fits perfectly now).
Many road calipers don't have you use a ferrule at the brake end. That's the intent here. If you want to anyway or if you must due to using a braided/compressionless brake housing, the solution is to use stepped ferrules such as Jagwire POP ferrules, which are long and have an opening for 5mm housing on one end and reduce back down to 5mm OD on the other end.