What ever will hold 3 days of clothing and 2 pairs of shoes. Every 3rd day get you clothing washed and change shoes besides any med's that is about all you need. Now between Now and Auguest make it 6 days (because you will sweat through 2 a day) and 3 shoes as they will be wet.
Depends on the time of the year, the countries, your interests... Do you need warmer cloth, jackets etc., are you going to take a larger photo equipment with you...?
Personally 75 litres would be an unimaginable boat anchor for me to haul around, but perhaps my style of travel is way different than yours. Here's my deal:
I travel independently for indefinite periods (several months at a time) via hotels/hostels through 3 climates with a 46 litre carry-on size backpack with LOTS of room left over - all the cinch straps are yanked to their tightest dimensions so the backpack is really only about 35 litres.
That's including a set of nice clothes to crash an Embassy party or high-end club, normal day-to-day casual and hiking wear, beach clothes, personal hygiene products and everything else you need to be safe/comfortable plus a laptop, digital still/video package and all the peripherals.
I have a separate daypack that fits inside the backpack. I use it to carry all my essentials when the main backpack is in an overhead bin, etc.
It's easily doable and you'll see LOTS of experienced backpackers doing the same - or even lighter. Seeing other travellers - especially smaller women - struggle with 60 - 70+ litre backpacks (plus additional huge daypacks!) is insane.
Going light makes sense to me because it allows me to keep my backpack with me (almost) all the time - it's carry-on size for aircraft, it doesn't need to be checked into the baggage space under the bus where it's out of my sight and out of my control - and it's obviously way easier/faster to navigate a crowded train aisle, keep with me inside the taxi, stow in a small boat, fit into a tuk-tuk, onto the back of a motorcycle and a million other situations.
Everyone's needs/requirements are different though... different strokes for different folks...
Good luck with your research and have fun shopping.
how big are you? ... 6'4" and 100kg or 5'5" and 45kg wringing wet?
We spent a month in Vietnam taking a small suitcase that fits into the overhead compartment of the plane and a small rucksack with a day rucksack. As we went mid Feb. to mid march we had only lightweight clothes with the exception of the fleece and zip off trousers that we flew in. Getting your clothes washed is very easy and cheap. No one dresses up in Vietnam even in 4 and 5* hotels and toiletries are available everywhere.
I have learnt during many years of travel in 3rd world to take clothes that are a bit past it and towards the end of the journey throw them away rather than take them back to UK. It also frees up room in your luggage for the beautiful items you will want to buy.
No need to lug the kitchen sink around with you.
The advice so far concentrating on taking only the minimum in my experience is correct. I personally do not like to get into a coach or taxi with my bags locked away from my sight.
A further advantage to travelling light is that you can buy clothing items so cheaply in VN if you have miscalculated.
Check out the Osprey Variant 52 and the Deuter Guide packs. These packs can be used for hiking and climbing and should be small enough to fit in the carry on. Deuter has a side pocket accessory that adds ten liters. You attach them to the side straps of your pack. These will give you extra space without adding bulk. Use them for water bottles or dirty clothes. Look in the acccessories section of Deuter. They should be about 20 dollars ot less and you can get two of them. One for each side.
Neither the Variant nor the Deuter packs are designed as travel backpacks, they're both top loading hiking packs. If you're doing a pile of trekking/hiking then they're great. For normal hotel/hostel backpacking travel on public transportation they would drive me nuts though.
They're also both way too big to meet carry-on luggage restrictions on most airlines.
If you get the under 40L versions of the Deuter Guide or Osprey Variant, they should be able to fit as a carry on. Then get two of these external pockets by Deuter and you'll have 55 liters. deuter.com/US/us/outdoor/external-pockets-39…
Top loading shouldn't be a problem. It means that you don't have zippers that'll get busted. And you can put the things that you need easily accessible in the top compartment.
The Deuter Guide 35 has a side zipper to access the main compartment. If you keep looking at websites it is possible to find them for under 150 dollars and especially toward the end of the hiking/climbing season.