I have a firewood saw horse that can hold wood that is too long and I can cut it. Take 2 2"x4" and bolt them together in the middle, swing them apart from eachother to form an "X" shape. Make 2 more of these and then take another 2"x4" and screw it to all three of the "X" pieces just below the bolt so that when the "X" is open it will stop at the right height. Make sure you space the "X" pieces out so that it will hold the wood and you can cut in the middle.
I use this to mainly cut branches but I also cut long pieces with it. Here is a picture similar to what I have made. I made mine so the pivot carriage bolt is more in the middle of the "X" pieces so I can get more then 1 log on it.
Also I have see people use something like this to cut long pieces:
or
. these might be better suited to what you need.