Sound advice from many, notably honourable mentions 34C and kristopher1805
Mate knowing where you are coming from (I was R/Cer and C/L aeromodeller for many many years prior to too) and ever the pragmatist I'll offer my perspective from when I started in model railway back in 2009. Rather a toy train set memory nostalgia inspired returnee than a hard core aficionado since in short pants, OO gauge was my choice then, and remains so now. Model rail is tactile and visual. Being older, for that reason I wouldn't go N then, argumentative disclaimer OMMV. However if you decide to go 00 gauge, do consider the following first.
End to end built along walls, a perennial favourite with seemingly endless -pardon the pun, options. Many serious model railway types derive pleasure from these. Other than a small/short modelling board, these don't suit my play value requirement.
Table tops. Whether using MDF or ply, most pragmatic to obtain materials to frame and build in our metric land will be 1200 x 600 sheets in your chosen thickness. They're a size facilitating easy handling and require no cutting other than handling and matching edge to edge over the frame. That's just a tad under 4' wide in the old money, and laid to whatever length your room accommodates. My smallest spare room (an air conditioned carpeted bedroom -comfort- which has built-ins, facilitating storage but reducing length) which is where I choose to play easily accommodates an 8'x4' walk around, but would squeeze in a 10'x 6' if I went to a central cutout control area with access flap and castors.
Sol's perspective earlier pretty much sums up the reality IMV&E. x 3' wide will do a fun enough shunting roundy, and you can even squeeze in a double track, but 8' x 4' is really the practical minimum tabletop size you'll want to facilitate a double oval for pleasurable (my value judgement) ops on a roundy. Don't do 3' wide would be my advice generally. Think again. It's far too constraining.
9'x 4' will facilitate operationally interesting roundy layout plans. Castors work fine on low pile carpet, even better on a concrete/polyurethane finished floor. Perfect if you can't access all sides. 8' x 4' or 9' x 4', the constraint is visual suspension disbelief length of express rakes. Compromise doesn't wear well on me, but given the choice of surrendering our HT room/Music Studio, compromise it remains. Living in hot 'n humid beside the ocean sub-tropical Qld, although toying with the idea repeatedly over the last decade of an dedicated shed for a larger layout, I arrive at the same decision on each occasion, discarding the idea even of one insulated and air-conditioned during play time.
GL with it which ever scale you decide. It's much more expensive hobby than RC, particularly now given our exchange rates for the Euro and GBP, but you can do it until you drop pretty much, at home without the travel expense or time/hassle, club fees or insurance nor the endless social BS and club politics that go hand in hand with R/C clubs. Cheers