You can modify the size of the tags by editing the file (Your Shopping Cart)/includes/display/SimilarProductsByTag.php
Search for the word $weight . It should be around line 35.
The code $weight = ceil(($tag['tagcount']/$tagCount)*100); determines how big the tag will be. If you want to make the tags smaller, then change 100 to 90 or less. e.g. Change the line to $weight = ceil(($tag['tagcount']/$tagCount)*50);
Once you've made the change, save the file and then refresh your browser page. The tags size should be different now. If it's still too big, or too small, you can keep changing the value until it suits you.

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