Needing advice about angle grinders...

[Little Rabbit]

To my mind, the sweet spot is a 5" (125mm) corded grinder.

7" grinders really hog the metal and are more or less essential if you're doing heavy fabrication. On the downside they get heavy, can be hard to control and to my knowledge, no-one makes the ultra thin cut-off wheels to suit.
4.5" grinders are easy to control, but always seemed a little toy-like to me and it can be difficult getting the full range of products such as cup wheels, flap discs, wire wheels etc.
5" grinders have plenty of power, with care you can do fine work, and seem to have the best range of products, where I live at least. I've always considered them be best balance of weight and power.

Cordless vs corded? Personally, I'd take corded; it never needs recharging, minimal electronics, you will never have to buy replacement batteries and (perhaps the biggest thing), if you cable tie the spanner to the cord, if never ever gets lost and is always located within 6". I don't think there's much difference in performance between corded and battery.

If it was my money, I'd buy two 5" grinders, and fit one with a ultra thin cutting disk and the other with a flap disc. Swapping discs can become tedious and inefficient.

FWIW while I've used multiple sizes and different brands in industry, my sole personal grinder is a 25 year old Makita 125mm which has run flawlessly. I treated it to new bearings last year and it turns out the original ones were perfectly fine. By the time I knew this I had already bought the bearings and pulled the thing apart...
I've used it for every thing from wood working to light fabrication to bone carving, and the only time I've ever missed a 7" was when dressing an anvil. In all fairness my work is usually workshop based, and on the odd occasion I've worked offsite, I've still been within a 20m extension lead distance.

Your kilometrage may vary.

Cheer E