I know this is a very old post but it was the first result when I searched for how to configure absMaxProducts so I think it's a good place to report my findings.
As far as I can see, this parameter cannot be set in backend configuration. But since I don't like to hack the code, I did some testing and I was able to manually add this value to the config table.
I.e. in table #__virtuemart_configs edit the field config add this at the end:
|absMaxProducts=10000
or whatever value you prefer. This has been tested on VM 3.0.18.