Grammar JLPT Level N4 - Lesson 22

そんなに, like そんな, is one of the most common ways to express that one thing is 'like' something else in casual conversation. The addition of the particle に just highlights そんな as being the 'destination' of the 'like' amount. This construction is regularly translated as 'so much', 'that much', or simply 'like that'.
As a word, そんなに is used adverbially. This means that it may come before any verb, い-Adjective, or な-Adjective.

• そんなに食(た)べたらお腹(なか)を壊(こわ) すよ。
  If you eat that much, you are going to break your stomach. (Natural Japanese for 'stomach ache')
• そんなに暑(あつ)いと思(おも)うならエアコンをつければいい。
  If you think it's that hot, you should turn on the air conditioner.
• そんなに嫌(きら)いなら、無理(むり)して食(た)べなくてもいい。
  If you dislike it that much, you don't have to force yourself to eat it.

Almost any そんなに based sentence can be translated literally as 'to an extent like (A)', where (A) is the word that comes after そんなに, and に indicates 'to'.
As そんなに is another example of こそあど言葉(ことば) (words like これ, それ, あれ, and どれ), it means that こんなに, あんなに, and どんなに may also be used.

• こんなにもらってもいいんですか？
  Is it okay to receive this much? (Are you really giving me all of this?)
• あんなに食(た)べたのにまだお腹(なか)が空(すい)いているの？
  You are still hungry, even after eating that much?
• どんなに頑張(がんば)っても、私(わたし)は先輩(せんぱい)みたいにはなれないと思(おも)う。
  No matter how hard I try, I don't think that I can become like my senpai.

Despite this, そんなに is almost considered a set phrase, and is therefore the most common out of all of the possibilities (except for どんなに, which is often used when explaining/giving reasons for something).