How does the size of the alkyl group in alcohol affect the vigor of its reaction with sodium metal?
The longer the alkyl chain, the more sluggish the reaction with the alkali metal.
Both methanol and ethanol would be very zippy upon reaction with sodium metal. Once the alkyl chain exceeds 2, or the alkyl chain becomes branched, reaction with sodium would become slow. Sometimes, the alkali metal can become coated with a hydroxide type gel, and the reaction completely ceases (of course, as soon as you put water in the gel dissolves and the reaction takes off.
Even neat isopropyl alcohol would be fairly unreactive towards sodium metal.