Learn French: Notre mur a une bonne épaisseur. - Our wall has a good thickness.

To say 'Our wall has a thickness of two meters', you would add the preposition de plus the measurement: Notre mur a une bonne épaisseur de deux mètres.

Are all physical dimension nouns like épaisseur feminine?

Yes, nouns for physical dimensions ending in -eur are feminine. Along with une épaisseur (thickness), you will regularly use la longueur (length), la largeur (width), la hauteur (height), and la profondeur (depth).

Why is the adjective bonne placed before the noun here?

Bon and bonne fall under the classic BAGS rule (Beauty, Age, Goodness, Size) for short, common adjectives that naturally come before the noun rather than after it.

What is the adjective form of épaisseur if I wanted to describe the wall directly?

The adjective is épais for masculine nouns and épaisse for feminine nouns. To say 'Our wall is very thick', you would say Notre mur est très épais.