First off, remember that Finland becoming a part of the USSR in 1917 would make for a lot of knock-on effects and butterflies. This is such a formative time in the Soviet state's history that things would be pretty different from the beginning. Just one immediate effect could be Finland going Red leading into the Baltics falling to the USSR as well through a north-south domino effect of sorts.
So, we probably would not have a WWII exactly like we did, and most likely the USSR ITTL would not fall on schedule in the early 1990s either - too much changed history in between. I think people tend to underestimate the changes to a TL such differences with small nations make. Finland going Red could butterfly WWII entirely, as it could conceivably butterfly Nazi Germany as well, etc, etc.
Generally, about Finland being a part of the USSR and being bigger than OTL, it would help the USSR in any war against Western powers like Martinus Paduei says. A sizable Russian minority in this larger Finland would be pretty much a given as well. We might also see a flight into Sweden by the Swedish-speakers in Finland, diminishing that traditional minority. Finland's internal geography would be different as well, if only because Viipuri would stay as one of the country's biggest and most affluent towns through the 20th century, the Finnish gateway to Leningrad. If this Finland ever gains its independence, it would be quite different from the OTL republic.