Slovakia military: 25,000 people.
Polish military: 300,000 people.
Czechia military: 30,000 people.
German military: 183,000 people.
French military: 270,000 people.
Basically, don't piss off Poland, you'll need to defeat about 500,000 soldiers, though do you really need to march on to France if you win against Germany? Even if you did, that's about 1/3 the US military, and nowhere near as well armed, or well trained, or well psychologically prepared. If you're Russia, about 1 million soldiers should do the trick; and that's not hard when you don't have moral qualms and 21 million military age men to throw at the problem; and (edit) possibly an additional few million from North Korea for purchase.
As for nuclear weapons; Russia can probably bet that using a nuclear weapon from inside France, or inside the UK, as a first strike, would be too controversial to even do. The government would probably be sued by human rights lawyers from inside itself for even trying; convinced that it's better to take the loss while remaining on the moral high ground. What's even the point, when NATO predicts they have less than 5% of the defenses required for the inevitable retaliation? https://www.ft.com/content/5953405f-d91a-4598-8b6b-6345452ca...
> (edit to reply): Russia isn't winning against Ukraine.
Yet.