Standard sprite size (tile of tibia size) is 32x32. A monster as big as a demon is 64x64. It is 4 32x32 splits of an image. Scripted to be solid on the bottom right square and the remaining are not solid. i.e. you can walk behind it etc.
A standard monster sprite such as a ghoul is 32x32 therefore shrinking the amount of images needed.
Tibia's perspective is very hard to get used to, and i would advise to start from editing an already made tibia sprite as it is very difficult to keep perspective correct. This is because ingame they look fine and normal. Yet on a graphic program i.e. PS or Paint.net they are tilted at an angle of about 45 degrees and tilted/twisted slightly.
Starting from scratch with no experience in monster outlining will be a lost cause. I tried it. I failed.
I have been spriting since january (items) and attempted monsters a few times and i was advised to start from editing already existing sprites until i master the perspective so to speak. And it is helping! So i suggest you try also!
You can always do 1 side (south facing/non-walking) sprite. I.e. facing you standing still. And see how it looks. This will be the base sprite and you can do the remaining viewpoints at the end. Also remember, i think the south facing is the west facing sprite at another angle and the north the east. So really you would only have to do half the designs and then just rotate them.
Good luck
& kind regards