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Hi maggie, Congratulations on buying your daughter a slow cooker. Its the same make and size that Viv (my wife ) and I have. Ideal size if you intend cooking a full chicken, duck or large piece of meat.
Go for the 6ltr one I would say, because the actual working capacity is listed at 4ltrs. You would need to fill it with liquid to the point of overflowing to reach the 6ltr mark.
Incidentally I would like you note the above replies from BBW and shaney as regards cooking without liqid in the pot.