Cutting a 4.5m long 2x6 in half with a circular saw
My normal way to deal with this is to put 3 scraps of 2x4 on the floor.  One is on the longer board near the cut point. One is at the end of the longer board, or at least halfway.  The third is not quit halfway down the short end. This gives you 1.5" clearance.

When you make the cut, the short piece is trying to pivot upward at the saw, but the sole plate keeps it down.  If you support it at the end, it will try to drop instead, and the back end of the cut will twist.  No big deal with dimesion lumber, but can be a problem with 4x8 sheet goods.

So example:  You are cutting a 16 foot board at 11 feet.  Make your mark.  One 2x4 goes on the 11 foot side of the mark.  No terrible accuracy required.  Leave an inch or so of clearance.

One 2x4 goes anywhere between 2 and 6 feet from the far end of the 11 foot section.

On 2x4 goes about 2 feet from the cut on the 5 foot section.

You make your cut with the bulk of the saw on the 11 foot section.  

If you cut a board to length with too heavy and unsupported end, you will usually get a triangular splinter coming off the supported board as you finish the cut.  For structural purposes this doesn't matter.  If you are making something that shows, you will care.

A chop saw often is on a support with entendible supports.  The supports don't have to be at the ends.