The Russian Tsar empire was nearing the end of its eastern expansion (would end in 1867 after the final expansion into what is now Alaska) when Napoleon invaded Russia. Either way Russia was massive at the time.
Was napoleons goal to just take Moscow and end there, or eventually push forward? If it was the latter was he aware of how much of a tremendous leap to take all that land would be.
Napoleon was perfectly aware of how big Russia was when he invaded. His plan was not pushing all the way up to Siberia (impossible to do in a single campaign even hundred of years later), but to seek decisive engagement on the border or, at the very least, not very far from Russian capital.
Napoleon's military doctrine was very starkly different in comparison to wars of previous epoch (17-18 centuries) where it wasn't uncommon to wage multiple relatively inconclusive battles and skirmishes. Without definite end in sight, wars could go on for years, sometimes even tens of years, with armies constantly reforming, attacking each other again, retreating and trying again. French Empire's war goals were much more precise. Napoleon's war philosophy usually included swift and direct attack towards main enemy army, its complete eradication (or at the least beating it to the level when it cannot continue fighting) and taking of key enemy positions and cities before they are able to react or set up new defences. Without further means to resist Frecnh invasion, the enemy would unconditionally surrender.
Roughly the same treatment was supposed to be given to Russian Empire during the 1812 invasion. Napoleon's aims were to catch Russian armies unprepared, use his numerical superiority and easily defeat them in detail. Without Russian army in the field, Napoleon would've been able to take Moscow (not de facto the capital of Russia, but still important city nonetheless) and decisively won the campaign. Napoleon didn't actually want to take Russian lands as his own or incorporating Russia into his empire, his goal was subduing Russian government and making Russian emperor do as he liked, that is: continuation of the continental blockade and support of France instead of Great Britain. There was no need to occupy the entirety of Russia when 1-2 key spots and no resistance from Russian side was enough.
But Napoleon couldn't catch Russian armies (partially due to their quick retreat, partially because of pure luck) one by one. Later, he couldn't completely destroy Russians at Borodino and Maloyaroslavetz. Despite Moscow and multiple other Russian cities being taken, emperor Alexander didn't answer any of Napoleon's demands. As time went on and it became clear that Russian army is not going to seek frontal engagement with the French head to head, and Russian elite wasn't going to surrender, Napoleon's position became untenable. He had to retreat, and his Russian campaign ended in failure. If he could quickly encircle and destroy opposing military in the first months of invasion, when there was still time, it could quite possibly become another successful endeavour, and Napoleon didn't need to conquer Russian territory or travel up to Siberia. After he didn't succeed in destroying opposing forces, and it became clear that Russian government was very stubborn, retreat was the only ending still possible.