That's not a bolt, it's a stay rod.
I'd say the longest bolt on a stock L series would also be the diff hanger bolt. Those ones holding up the engine crossmember are what I'd call studs, they don't have a hex head and are retained by a knurl/spline.
To properly answer Jaffa's original questions....
1) The largest bolt is the crank pulley bolt for sure. Don't ask me why I know this part well.....
2) I think the crank pulley bolt is the same for an MY, definitely just as big on an EJ motor.
3)The smallest bolt on an L? Well I suppose it would be an earth bolt under the dash somewhere... I'm sure I've needed a 6mm socket under there before so would that be M4? The little screw that holds on the choke knob in an MY would have to be the smallest fastener on the vehicle, but that doesn't count for an L...
Hope you don't mind, but I've just thought of a harder pop quiz:
1) What is the firing order on an EA81 (easy question to get started)?
2) What is the diameter of the clutch plate on a 2WD EA81?
3) What's the minimum scrap thickness on a rear disc rotor from an RX Turbo?
4) How many splines are on the inside of an inner front CV cup for a WRX?
5) How many blades are on the exhaust wheel of a VF10?
Answer all those correctly and I can make a harder pop quiz! BTW, they are all things I actually know off the top of my head, not stuff I've gone looking for.
Hopefully someone will ask a question thats really really hard soon, I like Subaru quizzes :P