Front antiroll bars on non-GTE/GSi models are 20mm in diametre. GTE/GSIs are 22mm (same goes for the tiebars). Rear ones are either 13mm (saloons and non SR models) or 18mm (SR/SRI/GTE/GSi/diesels).
Is your 1.2i a saloon? If not, then someone (other than Vauxhall) must have fitted the ARB because I'm sure the saloons were the only 1.2s that had them as standard.