how do all the Pokemon Essential game developers make those extremly cool buildings?
I know i have to make a lot myself but is there anything like a "building construction kit" or something like that?
If not, how to make custom buildings with the right size ratio?
All you need to make new buildings is to add them to an existing tileset or make a new tileset.
The easiest way is:
1) draw or find a building image you like (you can even printscreen an image from googlemaps/earth
2) open a copy of existing tileset with paint.net or photoshop if you have it.
3)paste your building in the tile on something you don't need or empty space
4)make sure the building fits perfectly in an integer number of small squares (copy paste small white squares with a red cross around your building or on another layer)
5) go back in rpg maker and press f9
6)create a new tileset by selecting an empty number on the down left
7)select the image you modified as tileset
8)create a new map and choose the new tileset for this map
9)draw with your new buildings
note that the new tileset will need to be considered as flat, you have to configure which tile can not be passed through or which one should appear above an other/ the character