What is the width of the cone fire breath of the Drakin (a homebrewed race)? I know it's however long, but how wide is it at the end of the cone? Is it wide enough to maybe hit multiple targets?
Cone-shaped breath weapons follow the rules for cone-shaped effects. These are found on page 204 of the PHB in the Spellcasting section. In particular, it reads
A cone's width at a given point along its length is equal to that point's distance from the point of origin.
Thus, it will take up multiple squares and potentially hit multiple targets.