"A larger" vs "larger" to describe the sample size of a study

Solution 1:

Grammarly detects size as uncountable in your sentence, and suggests that the indefinite article is redundant with uncountable nouns.

I don't believe that size is uncountable here: you are identifying one of a number of different sizes (just as one may have more than sort of beer, or water, or bread). In this case, I concur with you that Grammarly has made a mistake. It's only human.

When I tried it, I said its suggestion to remove the article was wrong and flagged it as incorrect. Grammarly now flags the sentence without an article as incorrect, for me at least: "It appears that an article is missing before the word larger. Consider adding the article."