Scrum halves tend to be about 5' 4" and stand offs can be up to 6' tall.

For me I would say that Stand offs tend to be more of an organiser and a good kicker whereas a scrum half tends to be more of a runner and takes the line on more

Like most answers: It depends.
Most teams now just play left and right, and either player could be in either slot.

The older way was to play scrum half at first receiver, closer to the ruck and so working more with the forwards, or to act as a link to the SO, who played wider and so more with the backs. Back then, the loose would also be more of a pivot and might play on the short side.

In a lot of ways, the left/right split gives you two scrum halves, with the fullback acting as the wide running stand off.

Leeds don't have a system. We just stick 3 or 4 on the pitch at the same time and see what happens.

As a few people have said, the most important part is to find a pair that work together

A perfect example is those two great Australian half back pairings of Wally Lewis & Alfie Langer and Peter Sterling & Brett Kenny

In the Lewis/Langer partnership, Lewis was the general who bossed the game while Langer had a more dynamic running game while in the Sterling/Kenny partnership it was the scrum half who was the general and the stand-off who was more off the cuff and dynamic. These two pairs complemented each other perfectly

(Of course Lewis could take the line on while Langer had a passing game etc but I am referring to the dominant role they performed)

The waters have got a bit muddier recently and the Cooper Cronk/Johnathan Thurston partnership for example doesn’t separate the roles quite so explicitly but the key thing is that all of the key skills and responsibilities are covered

A little unfair on the team and the coach there....Leeds run quite a structured system of Sinfield right, Mcguire left. There's a bit more play based around Burrow jumping out of acting half and a bit less of the [ifullback-standoff[/i role, but that's because Hardaker can't pass very well.

I was just being a tad mischevious with amount of playmakers we have.

Works though!

Not much difference between the roles in reality.

Paul Rowley played a strong tackling second rower (Craig Briscoe) at 7 in a game when it was raining heavily, he made 30 odd tackles right in the centre for an hour and then was replaced by Ryan Brierley who tore apart a beaten pack.

Just to show how cloudy the lines can be.

True most of the time it does, sometimes I feel we could do with fewer playmakers and a bit more grunt in the pack but I can't really complain at 6 titles in 10 years

Yeah youve got teams like Catalans playing Bosc on the wing for at part of the game, some teams have full backs coming in as halves, some teams have hookers, some teams use halves as extra hookers, some teams use back rowers as extra or stand-in halves. It all depends on the system the club wants to use.
From my albeit limited watching of the NRL I'd suggest that the halves in the NRL seem to play a more similar role to each other than the halves in SL.
Is that why they find it easier to fit into a national team?

It depends, the number on the back (6 or 7, not this squad number malarkey), doesn't mean much, Widdop will be the dominant half for the Dragons this yr whether he'll be wearing 6 or 7. Thurston is playing the same as he was when wearing number 7 for North Queensland, still trying to do too much. Ray Thompson and Robert Lui have to step in there and tell him he's doing to much, and let them take some control. Watched some of the match against the Penrith Panthers last yr, he's putting some of his team mates off.

Lewis used to be the dominant half, Johns used to be the dominant half, yet as you know one wore 6 the other 7.

A scrum half is more likely to cause a fight.

Great point it always seems to be a scrum half who starts fights.
(Also hookers do obviously)

One Usually has 6 on his back and the other has 7.

I guess it depends what the coach wants from their halves. Sydney Roosters play with Maloney on the left, Pearce on the right, and they share the play-making responsibility. North Queensland Cowboys have Thurston who basically takes both roles upon himself and his halfback partner Robert Lui just plays off him. Canterbury Bulldogs use their half-backs as second/third receivers and let their prop forwards take first receiver. It really does vary with every team. The definition of a stand off or scrum half is really want the coach wants from them.

I prefer the title 5/8th.