I’m echoing several of the points made.
I like fluid to slightly roomy fits in most of my knitwear tops all the time, which causes me to size up 1-2 sizes but doesn’t work in do every style, as noted.
But for slouchy, it’s usually a specifically boxier, shorter style. Also as per Suz, the sleeves need to taper/ banded so can push up. Also do not like the oversized balloon sleeves in any sweaters. In the boxy sweater, I might be true size and not need to size up.
Length has to be just right to hit correctly at hip and not be cropped/ midriff. Also if too tapered/ welted, they will ride up. If too weirdly boxy or fabric is not supple enough , it might stick out in front like a babydoll style. A lot of EF boxy tops do that on me.
I’ll give some examples. Last year I bought a couple VC exposed seam
sweaters- still available at Nordstrom & elsewhere. NOT the best knit as too much acrylic, but one color was especially good, I wanted slouchy and less worry than cashmere. Slouchy BUT a not good aspect was it bunched up extra slouch in a less than ideal way below waist. I had to size down act, to XS regardless. I wore it some with my knife- pleated skirts but that longer length & lower bunching looked too sloppy.
This year VC also has a “ cropped” version and despite still being acrylic blend, I bit, because it is the perfect length & shape for my skirts and for my high- waisted pants. Got XS, but needed S to be the right amount boxy for slightly pear shape.
Anthro has a cashmere sweater that is correct boxy / length for me.
A wardrobe hole is a mid- weight COTTON knit in styles just like those, for milder weather, & ability extend season with a light underlayer. Sometimes rayon/ nylon blends work. I'm trying a cotton one from Everlane.