Assuming one is eating let pizza than required to wash, what are the bracha rishona and the bracha achrona?
Let's start with some theoretical halacha: there is a food category called pas haba bekisnin, i.e. "snacky quasi-bread".
If one eats enough to call it a meal, s/he says hamotzee before and birkas hamazon afterwards.
Less than that (but still one kezayit), mezonos before and al hamichya afterwards.
Less than a kezayit, mezonos before and nothing afterwards.
Let's assume for illustrative purposes that it takes four ounces of snacky-bread to make a meal:
If I plan on eating four ounces, I wash and make hamotzee. If I then decide to stop after two ounces (I suddenly hear there's a big meal later and want to save room), I say al hamichya.
Conversely, if I think I'll only eat two ounces, I say mezonos. If after those two ounces I want another two, the Kitzur Shulchan Aruch writes that I make a new mezonos (as my first bracha didn't have this in mind; but not a hamotzee as I'm only now sitting down to two ounces), eat those two, then say birkas hamazon as I've now eaten four.
Your question assumed that pizza follows the status of "snacky-bread", which is a very common opinion, though hardly the only one. (DoubleAA quoted the opinion of Rabbi Welcher and others that pizza is automatically "real bread" every time.)
The definition of bread following the Halacha is a little different from the definition in litteral common language. According to the Halacha, to call (I mean to call lechem by Bracha Rishona) some food 'bread' refers to the function of this meal. In many verse (and also in philosophical language) bread is the pittance, livelihood, daily bread. The spouse of a man is sometimes called 'bread' (concerning Potifar "כי אם הלחם אשר הוא אוכל"). The main food is called bread. A food that has both features 1.- to be able to be the main course 2.- and to be perfectly suited for this, is called "lechem". It is worthy of bircath "hamotzi". A main course meet specific definition. In Mishna, Tanaym talk about this, regarding the Beracha Acharona (3 Berachoth / Meein 3). In the historical background in which Chachamim developed the Halakha, human feed is mainly made up of wheat and water. We kneaded and baked dough.
.In Italy, one of these forms is the Pizza (Note that in the geographic area of Rome, the pizza name also applies to the Focaccia which is composed only of dough bread oil and salt)
. But some Kosher pizzas have the mention that the dough was knead with juice, and/or that the Blessing is Mezonot "ברכתו מזונות" (Mezonoth). This hilchatic character will disappear if we decide to eat this pizza as our main course. Main course is something that we can not really attribute to any food. The original question is about a Pizza that we consider as "pat habaa bekissanin" (because it is knead with juice and the taste of the juice is very noticeable or because it is baked with a coating which makes it a little different from bread.). When the consumer decided to eat it as a real meal, it comes back to be bread, when it is eaten provides in a sufficient quantity as a normal meal .
Bracha rishona: Hamotzi; Bracha Acharona: Bircath Hamazon
 It is generally not considered the case in pizza because the ingredients that garnish the dough are those with witch one eat bread during meals OC 168, 17,Magen Avraham
 A big meal (Shaar Hatsiun 19) following the Shibolei Haleket (a quote from the Shibole Haleket. "ופירוש סעודת קבע השיב רבי אביגדור כהן צדק זצ"ל לרבי בנימין אחי נר"ו כי קביעת סעודה האמורה בכובא דארעא ולחמניות ופת הבאה בכיסנין הוא סעודת הבוקר והערב").
I was previously thought that we could understand the statement Shiboley Haleket as a Chumra (that is to say that the time fixed for the bi-quotidian meal gives the Keva's connotation even if the meal is smaller than 4 Beitsim) but Acharonim clearly understand it as a Kula, for Hamotsi and BHM we need a great amount (a voluminous meal such as lunch or dinner is also needed for Birkath Hamazon).