Learn French: La superficie de notre jardin est petite.

Both translate to "area," but superficie specifically refers to a two-dimensional measurable geographical or architectural area, like a garden, a country, or a room. Surface is broader and can also mean the physical top layer of an object (like the surface of the water). For a plot of land, superficie is the most precise term, though native speakers often use surface interchangeably in everyday speech.

How would I rewrite this sentence to state the exact measurements using the lesson's target grammar?

To state the exact dimensions using the "Noun + de + measurement" structure, you would switch to the verb avoir. For example, you would say: Notre jardin a une superficie de 50 mètres carrés. (Our garden has an area of 50 square meters.)

Since jardin is a masculine noun, why is the adjective petite in the feminine form?

Petite must agree with the core subject of the sentence, which is la superficie (a feminine noun), not notre jardin. It is the "area" that is grammatically being described as small, so the adjective takes the feminine "-e" ending.