Well, there's a couple of things that help there. For one, Derflinger is more on the gross messer side of the scale than the kriegsmesser. But I'm going to call him a war knife any way... admittedly because I think it's cooler. For another, and I usually don't tell people about my stealth edits, Louise is bigger than 1,50. Yeah. When I started thinking swords and Louise, I, well, I realized exactly what you're describing. Louise swinging around a two-handed great-sword (compared to her) will be a lot of fun, but I want to add just a dash of realism there. Then, there's the fact that Dutch and other northern Europeans (Dutch being the only ones I interact with regularly) are really, really tall. I don't know what they feed them but a Dutch woman starts at 1,65-1,70 and goes up to 1,85-1,90. Considering both the Duke and Duchess La Vallière, plus their daughters not named Louise, are around 1,85+ themselves, it's really weird if Louise stays 1,50. No, she's probably at the starting phases of a late teens growth-spurt and when she's 20 she'll be 1,90 (and so goddamn proud of it). So, I'm thinking Louise is almost 1,60 but can't tell herself because: 1) Not her first priority, 2) Everybody's taller than her except Tabitha, 3) Kirche is just that tall at 1,87 and 4) Massive Inferiority Complex and other Self-Esteem Problems.
Speaking of Tabitha, the French aren't exactly small people most of the time either. She's 1,55 and Louise thinks she's 1,40-something.
And on the other, third hand, a certain blacksmith in Tristania is having a fortunate but incredibly weird week.