It does depend on what I am eating with the above but as a guideline I usually have between 60-75g of pasta (I have cut this down gradually over time, used to have 100g), rice tends to be 50-60g, potatoes 200-250g, 2 weetabix or 40-50g porridge oats, wraps, petit pains and crumpets would be two (though if I am having a crumpet as a snack it will often only be one), baked beans about 1/3 of a tin but other types of beans probably 1/2. These are all the portions I would have if serving with a reasonable portion of protein and lots of veg. If I am still hungry after these portions I will go back and have more though and not feel guilty.
Hope this is helpful but it really is very individual and I would say if you are thinking of cutting down on portion sizes I would do it very gradually. Having enough protein at each meal is also a good way of feel satisfied without large portions of carbohydrates.