Learn Swedish: Det här är ett litet löv.

Because löv (leaf) is an ett word (ett löv). The adjective liten is irregular. When describing a singular ett word, it changes to litet.

Why does the sentence start with Det här? Could I use Den här if the noun was an en word?

When you introduce a new subject by saying "This is..." or "That is...", Swedish always uses the neutral pronoun Det or Det här. Even if the sentence was "This is a book" (en bok), it would still be Det här är en bok.

If I wanted to say "This small leaf" instead of "This is a small leaf", how would the words change?

It would become det här lilla lövet. When "this" is attached directly to the noun, the adjective takes its definite form. The definite form of liten is the irregular word lilla, and the noun gets its definite ending (lövet).

I have also seen the word blad used for "leaf". What is the difference between ett löv and ett blad?

Ett löv specifically refers to a leaf from a tree (usually one that loses its leaves in autumn). Ett blad is a broader word that can mean a leaf on a smaller plant or flower, or even a sheet of paper.