How to represent aggregates in a data warehouse
You should not change your date or pizza dimensions. What you need is a fact table that keeps your aggregate data. 

Therefore you will have Date dimension with these columns:

DateId -Pk

Day

Month

Quarter

Year

DateName

....

Your Pizza dimension will be like:
PizzaId -Pk

PizzaName

PizzaSize

PizzaType

...

And now your fact table which will be linked to the dimensions above using their IDs:

Create Table PizzaSale (keeps the aggregate data):
Id

PizzaId -Fk to Pizza dimension

SaleMonthId -Fk to Date dimension

Pizza_total_sale -Aggregate value

... Other aggregate values if needed 

So as you can see by joining these 3 tables you can get the number of sold pizzas per month. The type can be got from Pizza dimension.