Are RJ11 and RJ14 modular plugs the same size?

Do they physically fit the same jack?

The RJ14 jack can accommodate six conductors. If you use a single line,
four conductor phone cord (assumes a four conductor plug) into an RJ14 jack,
does it work OK (assumes the device the jack is in is a single line phone?

Will a two line, four conductor phone cord (assumes a four conductor plug)
work OK in an RJ14 jack (assumes the device the jack is in is a two line
phone)?