I'm guessing you don't want something military looking to not draw attention? The main two I use for hiking/camping are osprey and sierra designs, 55 and 80 litres.
Id go to an outdoor shop and see what feels comfortable to wear with your bike leathers/jacket.
Google search says your rif is 77cm long? That's too long for both my bags by about 5 to 10 cm, just measured them.