For flat or layered objects (paper, steel plate, fabric, cake) use 厚薄. For cylindrical ones (wire, rope, noodles) use 粗细. CC-CEDICT flags the reading hòubó for the figurative 'favoritism' sense (abbr. of 厚此薄彼).
mistakes
The character 薄 has multiple readings: báo (thin, as physical thickness), bó (literary/formal: thin, slight), and bò (only in 薄荷 'mint'). In 厚薄 the everyday reading is báo.
