Today I was most surprised by the lunch I had.

"the lunch" is about a specific lunch, but you haven't mentioned one yet, so it's better to be more specific in this sentence.

I went out to eat lunch, and ordered (lunch setfrom a set menu?).

Had to guess a little as to what you mean by "ordered lunch set" here

It was a unexpectedly big size chicken fries(or large) portion of (chicken with fries / fried chicken).

Because unexpected is modifying big rather than the noun, it's better to use the adverb unexpectedly.

Big is grammatically fine, but large is more common when talking about servings of food.

Chicken fries isn't a name of a dish. Not sure if you mean fried chicken (the chicken itself is fried) or "chicken with fries" (chicken with potato fries).

Today I was most surprised by the lunch.

the lunch -> lunch: "lunch" here is used as a label for the event known as "lunch", so "the" is not required.

I went out to eat lunch, and ordered the lunch set.

"ordered lunch set" -> "ordered the lunch set": here, "lunch set" is an object, which requires "the"

It was unexpectedly came with big size chicken fries.

"unexpected" -> "unexpectedly came with": I am assuming this "lunch set" came with multiple items, and chicken fries was one of them. Otherwise "lunch set" wouldn't be the correct term. Each part of the lunch set comes with the lunch set:

Example:
The burger came with the lunch set.
The chicken fries came with the lunch set.
The soda came with the lunch set.

So "came with" is necessary here. "unexpectedly" is the adverb form of "unexpected".

Today, I was most surprised by the lunch.

I went out to eat lunch, and ordered a lunch set.

If you are counting objects, for example, a lunch set, or two lunch sets. For singular objects add an article before the word like I did in the sentence above.

It was unn't expected big ing the large-sized chicken fries.

So, I couldn't eat them all.

I adjusted the previous sentence, so I had to add 'so' here.

Today I was mostLunch surprised byme the lunchmost today.

Starting with the subject since you are answering a prompt.

I went out to eat lunch, and ordered a lunch set.

It was unexpected big sizecame with an unexpectedly large amount of chicken fries.!

The use of "large" emphasizes the physical amount of all the fries you got. I included an apostrophe at the end because that is the "surprise".

Today I was most surprised by themy lunch.

I went out to eat lunch, and ordered a lunch set (combo?).

It was unexpected big siz did not expect huge chicken fries.