Alarm spell description mentions "Tiny" as a minimum size threshold:
an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area
It says "a Tiny or larger creature", not just "a creature". I assume, that means there are creatures beyond size categories, like flies or mosquitoes, which are too small to be taken in consideration in this case (and other cases, including combat, space control, etc.). What maximum size do they have?
Related question: How tiny is a Tiny spider?
Best Answer
There is no listed category smaller than Tiny.
Most likely the spell is worded this way so that the Alarm doesn't go off when you are approached by creatures that aren't really "creatures" in the D&D sense; insects and such.
The maximum size of these seems up to the DM, given that they likely don't have any stats at all.
(You'd be hard pressed to find any area with no creatures at all in it. And if you found one, you probably wouldn't want to sleep there...)