Hi Edward,
This is an interesting finding.
Actually, the min. and max. sizes of the left and right blocks are set at more than one place in Moodle's core code. They are set in all of the following.
c:\moodle\www\moodle16beta\moodle\course\format\topics\format.php
c:\moodle\www\moodle16beta\moodle\course\format\lams\format.php
c:\moodle\www\moodle16beta\moodle\course\format\weeks\format.php
c:\moodle\www\moodle16beta\moodle\blog\header.php
c:\moodle\www\moodle16beta\moodle\index.php
c:\moodle\www\moodle16beta\moodle\course\format\scorm\format.php
c:\moodle\www\moodle16beta\moodle\blocks\pagedemo.php
c:\moodle\www\moodle16beta\moodle\course\format\social\format.php
However, I maintain that you can over-ride the blocks' max size in a CSS file of the theme you are using, by adding these lines:
#layout-table #left-column {width: 500px!important;}
#layout-table #right-column {width: 500px!important;}
You can set width at any required size (500px in my example), even exceeding the set MAX_WIDTH parameter in the php files, it will work.
Whether you decide to go the CSS way or hack the PHP files depends on the final result you want to achieve.
Going the CSS way
You can fix the #layout-table #left-column or #right-column width throught your site or course, and this fixed width will be determined by your actual THEME.
Hacking the PHP code
You can change the BLOCK_L or BLOCK_R MIN or MAX values to achieve a result which will be dependent upon the actual contents of the blocks. The way I expect this to work is that e.g. if the "title" of one of your blocks is a character string of a certain length, the width of the column containing that block will be able to grow or shrink according to such string length (within the bounds you'll have set in the various php files listed above) or if one of your blocks contains an image of a certain width, etc. However, it does not work like this. If you put in a block an image whose width exceeds the BLOCK_R_MAX_WIDTH limit, the column will still grow in order to display the full image...
Now that we've got all the elements, you can experiment and hopefully achieve the desired result.
Joseph