Help Staub size
I was looking forward to buy a Staub, but not sure of what size...
I would be cooking for 2 regularly, but would like to be able to use it for more people as well... With a larger one, would I be able to use with smaller quantities also or it would not be as good as it should?
The other issue is either a oval or round one... The oval seems nice because of the possibility to have a lamb leg for example...
Oval versus Round:
As you noted, the oval shape is better for boned meats. They work very well with boned roasts and any odd shaped cut of meat you want to cook. Most meat is cut in a rectangular shape so, they fit better in a oval vessel.
Don't go smaller then 4 quarts as an only or first piece. Smaller sizes are good for side dishes but, really don't work well as the primary piece. A larger Dutch Oven works fine for smaller batches of soups, stews, beans, etc. but, you can't go larger. The Staub 5¾-quart 'Coq au Vin' Oval Dutch Oven is a good size "first" piece.
If you entertain a lot, the larger 7~8 quart models are nice for longer boned meats but, they just don't get as much use. Also, if you have any issues with strength, age, balance, etc. the ~8 quart Dutch Ovens can be too much to lift out of an oven when loaded with food. You really can't use an 8 quart Dutch Oven unless you are cooking once for a weeks worth of left overs.
Oval is more versatile, because like you said you can fit longer pieces of meat in the pan easier. I have both oval and round pots. I use my 6-7 quart range pots all the time for smaller quantities. They are great for browning meat in due to the higher sides containing the splatter. I'd say probably 1/2 the time I'm only cooking maybe 3-4 quarts in them and cook beautifully. Are they heavy, sure. But they are not more than I can lift even when full.
To fit a whole leg of lamb the oven would have to be very large, and that's not only super expensive, but it's super heavy. Heavier that one person can lift. Better to use a roasting pan for that and if you want to braise to use a layer or two of heavy duty foil crimped over the top of the pan to act as your lid. If you want oval I would go with the Coq Au Van which is 6 qts. If you want round I would start with the 5 or 6 qt. round. Be aware that Staub is even heavier than Le Creuset.
You can use smaller quantities in a larger one but there's more risk of the ingredients drying out or burning if there's too little in the pot.