If this "what-if" assumes a White on Red war without finishing Japan it is ridicilous. If such a war is assumed to start shortly after Japan is defeated then it depends on a number of factors:

Do the Western Allies demobilize? If they are starting this war as suggested this is unlikely so they would have about 90 to 100 mechanzied divisions in Europe with an average "slice" strength of about 35,000 men (ie including attached units) There would be about an additional 20 in the Pacific at the same strength. Additionally, China had about 10 Western equivalent and about 100 poor quality divisions in existance.
The Western Allies have complete naval superiority everywhere. Both sides have about equal numbers of aircraft but, the Soviet air forces are purely tactical in nature. Additionally, the Western Allies have an overwhelming preponderance of many technologies associated with air power, particularly nighttime operations.
The Soviets have about 300 - 400 divisions with about 40 mechanized ones with the leg / cavalry units running with slice at about 10,000 men and the mechanized ones at about 20,000 with slice. About a quarter of their units are in the Far East.
If it is the West that opens hostilities I would expect the initial operations to be very heavily attritive in nature with the Soviets taking very heavy casualties in "Kursk" type fighting. The Soviets would have also faced a secondary problem with their rear areas going into open revolt in many of the nations they recently subjugated. I would expect, in particular, the Poles to prove unreliable allies for them as would likely the Germans.
The Western Allies would also start to use strategic bombing to strangle the forward forces of the Soviets. Deprived of their rail net their Armies in Europe would be in trouble. The Soviets lack an air force capable of a like response and the Allies have more means to make up for lack of rail capacity (eg., shipping and trucks).
At best, the Soviets might initially push the West back to the Rhine exhausting themselves in the process. At worst, they make no headway and are getting ground into a pulp in the face of superior Western firepower. Note, the Soviets may have equal numbers of tubes in artillery but Western practice makes and their system of employment make them extremely vulnerable to counterbattery fire.
On the whole, the Soviets in the long run are in very serious trouble. They lack the manpower base to fight another long war having suffered what? 10 million plus casualties minimum already. The US has been virtually untouched and has just reached its full wartime productivity. Rearming the Germans would only help them. If the West can advance against the Soviets it is initially into friendly territory that provides more manpower and industrial capacity.
Stalin knew better than to start another war with the West and was right to avoid one.