So… I believe I have tracked this down and (hopefully) have a solution.
In the file includes/admin/class-wp-members-products-admin.php, at line 535 (in the current version), you’ll see this:
$('.wpmem-product-select2').select2();
Change that to:
$('.wpmem-product-select2').select2({width:"100%"});
That solves the mis-sized select2 when the value is empty (or at least, it should).
Alternatively (for those who might need things a little easier), there is a patched version from the current release here:
https://gist.github.com/rocketgeek/0874e0e5ed6af7254d462f39e3b3e9c9
To use the gist, copy that file to overwrite includes/admin/class-wp-members-products-admin.php
As a side note, I don’t recommend changing any plugin files unless it’s an official bug patch (which this is). I’ve already added this to version 3.4 in development, so when the next release comes out, it will include this fix.
Thanks for reporting it.
(If you apply the patch and find that does not solve the problem, be sure to let me know)