To stop a for loop early in JavaScript, you use break:

const remSize = [];
let remData;

/* ...I assume there's code here putting entries in `remSize` and assigning something to `remData`... */

let remIndex = -1; // Set a default if we don't find it
for (let i = 0; i < remSize.length; i++) {      
     // I'm looking for the index i, when the condition is true
     if (remSize[i].size === remData.size) {
          remIndex = i;
          break;       // <=== breaks out of the loop early
     }
}

That said, for this specific use case, you can use Array's findIndex (to find the entry's index) or find (to find the entry itself):

const remIndex = remSize.findIndex((entry) => entry.size === remData.size);

find stops the first time the callback returns a truthy value, returning the element that the callback returned the truthy value for (returns undefined if the callback never returns a truthy value):

const remEntry = remSize.find((entry) => entry.size === remData.size);

findIndex does the same thing, but returns the index of the entry the callback returned a truthy value for or -1 if it never does.

(There's also some function, but it wouldn't be appropriate here.)

More about looping in my other answer here.