Adding ceiling joists to workshop shed
For your new shed I would have built 2x6 stud walls @ 16''oc and the same for the roof joists. In Ontario, you will need 6'' walls to get a minimum of insulation. You can always fur out to get more depth for insulation if you have already built the walls but that won't change the strength of your walls. I'm not sure what the snow and wind loads are in your region. You should check that out. Those loads ultimately have to come down via the stud walls. Your existing roof is supported by a triangular structure rather than a roof ridge beam, which is fine.(rule of thumb; ties no higher than 1/3 the height). 2x4's should be fine to build up your 'truss', the bottom chord will be in tension as will be the vertical tie to the ridge beam as any deflection of the bottom chord due to the weight of finishes and insulation will pull on that tie. Normally, to create a truss, one would add diagonals from the bottom of your vertical tie to the centre of the roof joists, but if you tie your new structure well to the existing horizontal roof ties 1/3 the way up, the whole assembly should be sufficient.