Reaper -Piano Roll:
This screen shot shows us where we can change settings to help with note entry especially length of note. By changing grid settings we can change how long each note entry is.
By changing the value here from 1/8 to 1/4 the grid becomes a crotchet beat in length.
By changing the note length from 1/8 to 1/4 when we double click to enter a note it will automatically enter a quarter note or crotchet.
If we now change the grid back to 1/8 we have made the grid snap to 8th beats(quavers)