Backpacks ? I need help please

what about a 65 ?

I mean like its my first time so I may carry some extra stuff ,

would 65 be too big ?

and do you recommend any bags ?

Why do you need a 'big one'?

I've been to all those places,more than once.

I carry either a 30 or a 40l bag..dependning on destination and what exactly I want to do there.

For those countries I would use the 30l.....

BTW that is the same if its for 2 months,2 weeks or 2 years.

You need a few clothes...you do a little washing every day or every second day (like a t-shirt and underwear).

What do you want to put in an 80l bag?
How are you going to carry it around? On and off buses? Up and down stairs? Why?

I mean like its my first time so I may carry some extra stuff,

It's wiser to learn from other people's mistakes than from your own.

Leave the extra stuff at home and take only essentials!

http://www.travelindependent.info/whattopack.htm

30l - 40l bags are to small for me and the majority of people. Though I have used for trips before and thus why I now use a bigger backpack.

As you are going to warm weather countries and do not need cold weather clothing it means you don't have to pack as much. But when I buy a backpack I am not buying for 1 trip. I plan on using it for several years, in all different environments and climates. So pick a backpack taking this into account.

I travel now travel with a 70l backpack that is half full when I leave to warm countries and 3/4's full when going to colder countries. This gives me space for anything I buy when I am on the road.

I honestly do not know enough about brands of backpacks to help you out in buying one. I currently have a Kathmandu backpack and it has lasted 6 years, so far.

If by "backpacking" you mean traveling from hotel to hostel something in the 40-60 liter range should suffice.
If you mean spending an entire week on the trail, carrying food, water, fuel, sleeping pad, tent, ground tarp, map compass, shovel, toilet paper, soap, toothpaste etc. and resupplying only every 5-7 days you will need something in the 65+ liter range.
http://www.rei.com/learn/expert-advice/backpack.html

Personally I recommend getting a larger one, just in case 2 years from now you come up with new travel plans and decide to do the trail thing. Others here disagree and have vehemently slapped my wrist just for recommending the larger ones.

As you can see Lancaster, different people have different perspectives. The adage goes pack half of what you think you need and take twice the money. The smaller the bag the better for ease of travel. Having said that, I think the mid-sized are the way to go. I, personally, don't like having to wash a t-shirt and some boxers every day or two, especially as this may result in clothes always being damp if they can't dry (an issue in humid places especially). I prefer to have enough clothes for 1 week - 10 days and give my laundry to someone to do (which is normally pretty cheap in most places, though of course at times a laudromat or hand-washing is the only option). That means that something like 65 or 70L is normally what I go for when I buy a new bag. The key thing is to try not to fill the backpack just because you can, as gerard says it's good to have some free space.

There are other things to consider - for instance I like to have a bag which has an opening on the front meaning it's easier to access your stuff than one which only has an opening on the top. Do you want to have dividers or just one big area? What about side pockets? The possibility of attaching straps to the top?... The best is to go to a shop and have a look around at what appeals to you.

I'm with number1 on this question.As small a bag as possible,and as little in it as possible.
Save money,save your back.
Why carry around a big bag half empty? To go shopping?