Length of class/"double" classes

[Matt]

No: HPFGUIDX 79667

--- michaelkgidlow asks:
> I always assumed a standard class was 45 
> minutes, but in book 5, both a single period 
> and a dubble period are refered to as being 
> one and a half hours long. What's up with this?

I was thinking about this yesterday when I followed some link at the
Lexicon to a class schedule.  You (and the Lexicon) have interpreted
"double" as double in length.  I thought that it referred instead to
doubling the *size* of the class (q.v., "double Potions with the
Slytherins," "double Herbology with the Hufflepuffs").  If there is
anything in canon that conclusively resolves the difference, I missed
it.