I think I was talking about them both, but then being more specific about Helen (as I am not aware of any canon in relation to Harriet's bum!). ylla has identified the Helen quote. For Harriet, published canon is vaguer - we know that she is slender (but not, I think, slight - there are plenty of references to her strength, so she's probably not got a tiny frame), tall, possibly a bit bony.
The TD "what a figure" bit is DLS, from memory, but Sayers' draft - the one JPW had access to - actually has a brief Gerald POV moment in which he considers that she's decent-looking in her way, and specifically has him thinking that Harriet's bosom is on the small side, but acceptably shaped! (It's actually a rather better moment than that makes it sound, because there's an interesting character bit with Gerald and him trying to build a relationship with Harriet and to like her, because he cares for Peter and wants him to be happy, and I think that JPW was daft not to include it.)
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The TD "what a figure" bit is DLS, from memory, but Sayers' draft - the one JPW had access to - actually has a brief Gerald POV moment in which he considers that she's decent-looking in her way, and specifically has him thinking that Harriet's bosom is on the small side, but acceptably shaped! (It's actually a rather better moment than that makes it sound, because there's an interesting character bit with Gerald and him trying to build a relationship with Harriet and to like her, because he cares for Peter and wants him to be happy, and I think that JPW was daft not to include it.)