I'm planning to buy a new carry-on for my regular business trips (mostly weekly, 2-3 days each time; could have some longer ones but 95% of the time no more than 4-5 days each). I always just do carry on, and am now debating between the two size options from Rimowa - 56x45x25 or 55x40x20. the difference in storage volumes of the two is quite big, some 48L vs. 30L. leaning towards the bigger one for the better flexibility in case I need more (although I think for 70-80% of my trips I only need the smaller one).

they call the smaller one "IATA", although I thought the official IATA carry-on size is the bigger one. but, I also think the smaller one would be ok with most likely all airlines, but some airlines (maybe half?) would have carry-on limits smaller than the larger size above.

my question is - if I buy the larger one, based on your experiences with various countries/airlines, how likely am I to encounter problem (i.e. be asked to check in my luggage) ? I travel business/first class 95%+ of the time if this is relevant (or maybe airport security don't care).

my travel is mostly within Asia (with CX/KA/SQ mostly - I think I'm relatively comfortable with these airlines I'd be ok with the large one), but sometimes with LH/BA/AA/UA - how strict are these airlines in terms of carry-on limit ? and how strict are the airport security, e.g. at LHR or FRA/MUC ? (I assume in the US it's probably ok since I always see people having big carry-on luggage anyway?)

thanks a lot for any advice you might offer !!