Clothing and shoe sizes vary between countries in Europe. Now, if you are a typical UK shape and size, be prepared to buy most clothes in Italy in size L or XL! Do not get harassed by this, its just the way it is. Take a look around you at the Italian people and you will see that many of them are about a size 8 at the most ranging from the teenage kids to the older generation!
In my experience of clothes shopping in Italy, trying on jeans here is what I would imagine to be the equivalent of forcing your legs into the sleeves of a well fitted shirt (no I am not in the habit of trying this!) I did eventually find shops that catered for someone a bit larger than a twiglet! As with any clothes shopping, no-one sees the size label and make sure you buy something that fits you well and feels comfortable on, this in itself will leave you feeling great.
Remember that clothing and shoe size can vary depending on the designer and make of clothing. Always try before you buy.