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Hi Nikita
Looking at Zimin's key (Keys to the Insects of the European Part of the USSR, 1969), I think that the fore tarsus of intermedia should have segments 2 to 5 reddish yellow all over, not just on the ventral side. Also if you compare figs. 3 and 4, there is a difference in the curve of M1+2. In intermedia it seems that the vein is curved closer to the wing tip. In meridiana the curve is more basal. It would be good to see a photo of intermedia to check whether this is a good character. Perhaps Kahis has a specimen and could take a photo of the wing for us?
As far as size differences are concerened, remember that muscid males are almost always smaller than females.