Please provide us with a hand drawn floor plan of this area plus the room itself and measurements. A modular couch that comes apart with very narrow arm rests may be one answer (and nothing "oversized"). Items in a store look smaller than in your home. As always - measure (all areas and the furniture) twice and buy once. Is there an outside door to this room? Also, what is your style? If you want to discover that, put in top HOUZZ search box (one at a time) different styles. Examples include transitional, traditional, casual, old world, country, eclectic. It also depends on how the furniture is designed. I have a dining table that has removable legs.

Also let us know what is the purpose for each room you needed furniture for so we know what kind of furniture, guest apartment, bedroom, living area, office, TV watching, conversation, bowling, crafting, sailboat building, ginsue knife sharpening, tchotchke display etc.. lol...

Not sure what you’ll be using your basement for but we got the Ikea Kivic 6 seater sectional with 2 storage ottomans so everyone has a cozy lounge spot (it holds lots of teenagers) - because its modular it was super easy to get down our super narrow basmenrt stair - we have a sharp 90 degree corner at the bottom also so special planning is needed for sure

I concur with @la_ la Girl - IKEA products that are assembled in place make getting items into the basement easy, or building in place. Our access is from the 1st floor hallway but the stairs make a 180 degree turn at the landing.

Here’s a video of ours. Most of the cabinets and furniture are IKEA - they have functioned well.

https://youtu.be/3FqMEqaf0-I