You can scale sprites to 64x64 using this approach:
This tutorial was made for a reason to support newbie OTS devs. I am not owner of any tools or solutions done neither I really promote to use OTCv8 but nevertheless this OTClient fork have already built-in everything you need regarding the support of upscaled sprites so you can investigate or play with them further. There are countless threads in support board of how to use upscaled sprites in the game client and if you were looking for a solution you are lucky cause this tutorial is for you.
Prerequisities
If You are not happy with the results then You can try any upscaler available online
In case You are still not happy with the results - im selling scailling service - You can PM me on discord #mrianura