Why? The center voltage vo1 should be around VDD/2 (to achieve high swing), so you need that high shift 'cause VSG6 shouldn't get too much overdrive...
Actually such a current source produces a PTAT voltage reference, which - by comparison with a second PTAT voltage reference - can be converted into...
By W/L ratio (=1) between (P3,P4):(P1,P2) and (N3,N4):(N1,N2) resp. Don't forget: V2 & V3 must be supplied externally. Also, the common drain voltage...