copper reducer size "does it matter"

[snuffy7]

I'm using 2" copper from the top of my keg until I reduce it down to 1/2" going to my leibig. I see a lot of guys using several reducers (example 2" to 1" to 3/4 or 1/2 etc.) Is this for any certain reason or just not being able to find a 2" to a -1/2" reducer? Will mine work ok?

[instableoxymoron]

How long would a 2" pipe need to be on a 15.5 gallon keg? I was also thinking of reducing the 2" to a 1 1/2 inch prior to the 90 degree turn (saves me $5) and then eventually having a 30mm liebeg condenser made with 3/4" pipe and 1/2" pipe.