For flat or layered objects (paper, steel plate, fabric, cake) use 厚薄. For cylindrical ones (wire, rope, noodles) use 粗细. The reading hòubó marks 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.
