Even for an hungry italian a complete menu is too much! In fact, lot of restaurant suggest some "touristic" menus and all of these menus exclude a part of complete menu; for example are Starter - First course and Dessert or First and Second course and Dessert.
Restaurants manager know this so, when you eat "alla carta" (ordering what you want from menu) they usually take an ordination step-by-step, for example, for starter and first course then, when you finish your first course they ask you if you want a menù for seconds or dessert or if you want just a coffe.
If you order a first course and your partner orders a second course you should ask if you want to receive all together or if you want two dishes to eat half and half of everything, there's not a rule on this...
For quantity of meal in the dish depends a lot from the restaurant, normally more the restaurant is elegant and expensive, less is the meal in your plat!
Usually, when I go into a new restaurant, I check the dishes around me to understand their dish's size.
IMHO best restaurants are "osterie" which, usually are "rustic" restaurants in secondary streets with paper tablecloth and a restrict menu. Usually are cheaper restaurants with generous plat of local and fresh meal.
For condiments I've never had trouble, normally they all have a good measure...
For what to eat depends on what you like and where you are; for example, here in Bologna, we're strong with salumi, so starters, we've got a lot of good first courses and dessert but we're not particularly famous for seconds, so in Emilia Romagna I suggest you starters and first courses, in Napoli pizza, in Roma first courses again, ecc...