Was in exactly yr position 2 weeks and did a fair bit of research before grabbing a 3.5l – found it suits two of us fine. Bigger is not always better if it takes up yr work surface and freezer.
We always get at least 4-6 meals out of one cook. Decided via reviews and not disappointed. Readf that more gadgets ie autocook etc were not necessarily better and often worse. We got one with high, med and low. If feel I need auto ie will be out all night have decided to just use a wall socket timer thing.
Morphy Richards 48702 3.5L Sear & Stew (Was 25 quids from a well known automated warehouse store)
Best thing is you can use the metal inner to brown onions etc on a hob before returning to slow cooker. Does not look as nice as ceramic to serve tho, as others have said – but we serve in other things or usually straight to plates or freezer containers.