The old standard was a "big" bag and another whose description included "a briefcase". For around 15 years I have gone long haul 4 times a year with BA in C or F. I used a slightly smaller "big" bag and something akin to said "briefcase" with laptop, business papers in it. I am usually surrounded with people carrying the same combination. There is plenty of room for everything. The position in Y is, I'm sure, different and that is where the problem lies. So why this change targeted at all classes of travel? It is guaranteed to p*ss off BA's premium pax no end.
There always seems to be enough room in European C as well. This change brings BA into line with EasyJet and thus, at a stroke, removes one of their USPs.
And I am still struggling to interpret the change to the boarding arrangements. Previous practice was Fast Track for F, C, and Executive Club. New practice is to board by cabin and in order of Executive Club status. So are they now going to call all F by status, then C by status, the Y by status? Or what? Speaking personally I pay a premium and expect to be sitting in my seat rather than spending an age in a queue. But maybe I am not a typical passenger.
I'm sure someone at BA has spent a lot of time determining that these changes are going to "improve the customer experience". I guess in doing so they got carried away and failed to consult their (premium) customers.