Hey there.
I know this is a late reply, but I've seen this question asked a few other places so I thought my solution my help others.
In the file: wordpress-seo/inc/wpseo-functions.php - around line 177 (search for 155), you will find two instances of '0, 155'.
Simply change both of these to '0, 141' - or whatever you want. I set mine to 100.
Then:
%%excerpt%%... %%ct_product_tag%%: Learn more. Buy online.
creates a pretty neat meta description around 150 chrs with a nice bit from the description, the brand, and then a little sign off line. Works pretty good for a large catalogue as a fallback default.
NOTE: you probably will need to make the changes whenever you update the plugin. Don't know if there is some way to avoid that.
Hope that helps.
Cheers.
MQ
