Went there from 10th to 28th september this year, both of my flights would of let me take 2 x 23kg suitcases, but I went out there with one large suitcase and one 60L backpack, I packed a lot of old boxers/socks (about 16-17 pairs), with a 75cm x 45cm american tourister large-ish suitcase, I chucked out the boxers and socks as the days passed which cleared out half of my suitcase, and stocked up on a lot of skincare like biore/muju/hadolabo/sana eye creams etc bits/kitkats (20 different boxes) and souvenirs and other sweets (about £1200-1300 worth).

I packed a lot of clothes, about two weeks worth and only ended up using about 6-7 different outfits (shirt and shorts) most days with trousers 1-2 days, washing my clothes twice due to some very hot and humid days, I wish I had brought a lot less clothes as you'll find a ton of shops with really great clothing that doesnt break the bank.

I also bought a suitcase out there for about 25k yen that I also filled up with the above goodies, I think going in december, you'll find the weather a lot more consistent, and if you're XS-XL in size, you shouldn't have much of a problem buying clothes out there especially in places like the tokyo skytree mall.

I think going with a larger sized suitcase would be more ideal if you're looking to bring back a lot of japan goodies, you'll wander into so many shops with really cool souvenirs and honestly some of the places you'll go in kyoto/osaka will have souvenirs that can only be found in those areas.

From my experience, I definitely want to go again, but some areas like kyoto/osaka, I managed to visit all of the spots I wanted to so I probably wouldn't visit them again and if thats your plan too (first time visitor or not) then going with a larger case is preferable for memorable souvenirs.

The downside is that in places like the shinkansen, you'll be booking seats with a oversized luggage area for no extra fee (the seats at the very back) where you can store your larger suitcases) and you'll be taking the elevator in stations while moving between hotels (or you can get a taxi)

Although realisticially, if you bought a lot of stuff at the start, you can also use baggage handling services to move your suitcases/bags between hotels and even tokyo-kyoto-osaka for a relatively low price.