Depends.
If you want fake 64x64 then:
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...
otland.net
(It works perfectly, and dont get me wrong- it's really great! i call it "fake" as you need to use 32x32 tibia.spr for.. for example RME & got other file for the client; personally i would go for it if i hadnt made 64x64 10.98 tibia.spr before this tutorial was published)
If you want real 64x64 then its a bit more complicated:
1. Extract tibia.spr
2. Upscale all the sprites using, for example, PowerToys or any other graphic tool that allows bulk resize of images (it's just a matter of minutes on average pc to upscale them all)
3. Download object bulder
4. Load ur tibia.dat and tibia.spr
5. On the first window select 64 64 + extended
6. On the right window replace every single 32 32 sprite to 64x64 (the best option is to run macro to replace each one of the sprites & ignore blanks)
7. Save & Done, you got ur 64x64 tibia.spr
8. Download special RME & item editor for 64x64 tibia.spr
9. Compile OTClient (i recommend v8) to work with 64x64
10. Ur done and ready to play in 64x64
You may check this out:
Hi all, Here's a link to tibia.spr with 64x64 sprites (which I said I will share in a different thread so I am sharing). It's not, sadly, completed in 100%. There still might be some 32x32 sprites in the file. I would say it's around 95% completed. 615.76 MB file on MEGA...
otland.net
I could set everything up for you and convert ur 8.6 tibia.spr to 64x64 (its easy) but theres no way i will do it for free due to the time-consuming process of replacing sprites.
Fingers crossed one day someone who knows how to code will write compiler for .spr in 64x64
//edit
When it comes to sprites larger than 64x64 - i dont recommend doing this using the 2nd option.
Not sure how this would work with the first one.