SAR (in h264) means Sample AR, not Size AR. See
here. I don't remember exactly its definition, but it describes the aspect ratio of the pixel. A SAR of 1:1 corresponds therefore to square pixels. (According to some sources, SAR was previously called PAR, Pixel AR.) AFAIK, in the AVC streams of a 2D BD, the SAR is always 1:1.
It seems logical to assume therefore that Half-SBS would require a SAR of 2:1, but it's not the case. Most, if not all TVs and hardware players require 1:1, even if the pixel is rectangular. It's the value used by BD3D2MK3D.
The value to use is not clear for Full-SBS or Full-T&B. Stricto sensu, they have square pixels in the h264 stream, but some players (notably most LG TVs) require respectively 1:2 or 2:1. I have never understood why. Unfortunately, BD3D2MK3D cannot know that, and I have adopted 1:1 by default, but this can be changed with Settings -> Full-SBS/T&B Aspect Ratio.
To make things even more complex. the AR in the MKV header describes the size of the picture. It its value is easy to figure out for a 2D movie (16:9 or 1920:1080 for example), the value for Half and Full SBS and T&B are less straightforward. Is it the size of the two joined pictures (as saved in the MKV) that matters, or the final size of a single view? These 2 sizes are 16:9 for Half-SBS and Half-T&B, but the width or height is 2 times as big for Full-SBD and Full-T&B. Again, the value to use depends of the player, and although it seems that 16:9 works for all players for Half-SBS and Half-T&B, it's not as easy for Full-SBS and T&B, and again, you have to check yourself the correct value for your hardware player.
Note also that many TVs (notably most Samsung TVs) assume that all 3D movies are in 16:9, and ignore completely the SAR in the AVC stream and the Picture AR in the MKV header. They stretch the picture to occupy the whole screen anyway. (It's why it is a very, very bad idea to crop the black borders of the 3D movies.)
This is extremely confusing, also because some TVs use the AR from the header for AVI movies, but they prefer to use the AR from the video stream for MKV files. I don't understand why they use different methods just because the container is different!
[EDIT] tebasuna51 has replied before me.