I wonder how large it is size wise compared to an OB6 or P6.

According the Sweetwater site...

Hydrasynth:

Keyboard: 31.5" L x 13.63" W x 4.53" H (80 cm x 34.3 cm x 11.5 cm)

Desktop: 17.4" L x 8.78" W x 3.39" H (44.2 cm x 22.3 cm x 8.6 cm)

OB-6:

Keyboard: 31.8” L x 12.7” W x 4.6″ H (80.7 cm x 32.3 cm x 11.7 cm)

Desktop: 20.75” L x 7.8” W x 4.4″ H (52.0 cm x 19.8 cm x 11.2 cm)

Prophet-6:

Keyboard: 32.3” L x 12.7” W x 4.6″ H (82.0 cm x 32.3 cm x 11.7 cm)

Desktop: 21.25” L x 7.43” W x 4.4″ H (54.0 cm x 18.9 cm x 11.2 cm)

Conclusion, they are pretty equal size wise.