Wow, this is extremely early to plan!
Can anyone recommend any cities or countries?
Where did everyone usually stay? Hotels? B&B’s, Hostels?
I could recommend some cities or countries but they would be what I wanted to see, and pleased I saw them, rather than your own desires. For example I love Adelaide but a lot hate it. A lot of people love Maguluf but I hate it. Its your storey, write it with your own words. Do some research, watch some documentaries or whatever to see where you want to visit.
You will meet other travellers while away who will advise you in cool places they have been and your plans nay change, so keep them as fluid and as broad as possible...
Rough figure of how much you took?
I worked while I was away but took £5000 yo Australia for a year.
How did you start to plan your trip? Did you work out where you were staying before hand or just wing it when you land?
I worked out my first accommodation as it was my first big trip alone but you can easily just wing it. It's down to different personalities, again like countries, what works for one doesn't work for another. If its your first trip I would suggest booking a few nights in your first accommodation and go from there.... what's the worst that can happen?!
And also last one.. luggage size?
Again, different for different people, with different plans and destinations. You will need to take more on a trip if your trekking in Nepal, compared with hanging around the East Coast of Australia.
A 60L pack will probably suffice, but it depends where you want to go and how much you want to take. Sounds obvious but is critical in determining your pack size.
Seriously though, you have ages until the trip. If I was you I would concentrate on saving as much money as possible. In the last 8 months that I've known I was going to NZ, I've restricted drinking and going down the pub as this drains my cash!
I would also DL a bit research for the kinds of places you want to visit and the experiences you want to have. Don't leave it down to an online forum to decide every destination you visit...