Learn Finnish: Pikku hame on nätti. - The little skirt is pretty.

Both words mean "small" or "little", but pikku often carries a more affectionate or cute nuance. More importantly for your current grammar topic on dimensions, pikku is an indeclinable adjective!

What does it mean for pikku to be an indeclinable adjective?

It means its ending never changes to match the noun it describes. If you said "I like the little skirt" (Minä tykkään pikku hameesta), hame takes the elative case ending, but pikku stays exactly the same. If you used pieni, it would have to match the case: pienestä hameesta.

Why are pikku and hame written as separate words here?

Pikku is frequently attached directly to a noun to form a new compound word, such as pikkuhame (miniskirt). Written as two separate words here, it simply refers to a standard skirt that happens to be small, rather than the specific clothing category of a miniskirt.

Why is the describing adjective nätti in the basic nominative form instead of the partitive?

When describing a subject with the verb olla (to be), the adjective takes the partitive if the subject is an uncountable mass noun, like water or coffee. Because hame (skirt) is a whole, countable object, nätti stays in the nominative case.