Here is the size comparison between the three 85mm FE lenses. In that case I have no doubt the smaller Sony f/1.8 FE lens is the best choice unless you really need that tiny faster aperture. The GM is the bokeh king while the Sigma is the sharper than the GM. But I think the Sigma is really way too big for my taste :)
I do own the 85mm f/1.8 and it’s super for my hiking trips (light and sharp). Get it!
Links to the three 20mm lenses:
Sony 85mm GM at [shopcountry 71598] Sigma 85mm at [shopcountry 71597] Sony 85mm at [shopcountry 71599]