Hello Ahoikoi
First of all, congratulations on buying this incredibly great filter machine; we couldn't be without it!!!!
For "4-6 cups" we use the 1x4 size from Melitta. For less, we use the smaller ones...
But it has also happened to me that the smaller filters "overflow", which is not bad, but causes a bit of a mess. Accordingly, it is better to use larger filters and possibly cut off the rim at the top and "compress" the filter in the tank a little.
Have fun with the Optima Timer😊

Melitta filter bags size 102