Hello. Had to increase our supplier_reference characters recently for when importing CSV and displaying on front office product page. Found all the info I needed to make it work but in various places so I thought I would share what I changed in one place;
1) Classes > Product.php (Line 458. Change end number to what ever you need).
2) Classes > ProductSupplier.php (Line 71. Change to the same number as above).
3) PHPMyAdmin > ps_product (Click the + icon to the left of 'ps_product' and then the + icon next to 'Columns' that appears. Click 'supplier_reference' at the bottom of the next appearing list. Then change the 'Length/Values' on the right to the same number as you did for product.php).
4) PHPMyAdmin > ps_product_attribute (Click the + icon to the left of 'ps_product_attribute' and then the + icon next to 'Columns' that appears. Click 'supplier_reference' at the bottom of the next appearing list. Then change the 'Length/Values' on the right to the same number as you did for product.php).
5) PHPMyAdmin > ps_product_supplier (Click the + icon to the left of 'ps_product_supplier' and then the + icon next to 'Columns' that appears. Click 'product_supplier_reference' at the bottom of the next appearing list. Then change the 'Length/Values' on the right to the same number as you did for product.php). <--- This one will increase the character limit in supplier_reference when creating/editing a product in the back office (BO). Thanks to ChatGPT for this one! 
I hope this helps someone 
Cheers.