"This desk is big."
Translation:이 책상이 큽니다.
책상 ends in a consonant so you must use 이 to make 책상이. Words ending in a vowel can use 가 such as 사과 -> 사과가. 는/은 would be used to mark the topic and 이/가 is used for the subject.
I'm confused by the 이 being both before and after 책상. I'm a beginner so maybe I'm missing something obvious.
The first one means "this" and yhe second one has no meaning its "the subject marking particle" used to make the subject more obvious to the listener