I have various constants (e.g a constant uint8 TileSize) that I need to access in my c++ code as well as in my blueprints. Where should I best store such data so that I can modify it from editor (if possible a big plus but not extremely important) and can access it from both c++ and blueprints?
You said you want to edit default values on the editor.
For that you need an asset containing a default object…
Otherwise I would just create a static class and a function library to change static variables from Blueprints when needed. And forget about editing defaults from editor UI.