I'm a similar size to you OP. I love the oversized and slouchy look, but at my height I just can't do it, I look overwhelmed! I need clothes that fit, a size 12 jumper is as oversized as I can go.
7/8 length cigarette trousers or jeans work well, and as PP have said, and a T-shirt that's not too loose. I wear a lot of cashmere jumpers and cardigans, a cropped length is good for me but hard to find. Paperbag or boyfriend jeans work but only with a fitted top tucked in. I have a few fine knit merino jumpers that I would wear with them.
I love pretty white blouses, Zara is usually good for these, they make a nice alternative to the white t shirt, I just feel like I've made more of an effort when I wear one.
I also have a couple of blazers that fit really well, one is jersey so looks less formal. A blazer that fits well on the shoulders is a brilliant thing to have in your wardrobe, it makes any outfit look good.
With skirts, I usually go for an A line, shorter skirt. I love these and have a few that I wear with opaque tights. I much prefer these to wearing trousers and they go with the same tops.
I love the look of all the midi dresses around now, M&S and Warehouse have some lovely ones, but I have to be really careful not to be swamped. If there is a petite version I'll go for that. I would always belt a dress like that otherwise I feel like I'm wearing a tent. Also a blazer works well to give the outfit some structure.
I would go to somewhere like John Lewis, give yourself a few uninterrupted hours and try on lots of styles. Trousers and jeans are very hard to buy online I find, the fit is so important.