。
Ryoko has this cute little compact face.
Literal
Ryoko-san [subject-が] snugly [adverbial-と] did face [object-を] is-doing.
ちんまり is a state mimetic meaning 'snugly, compactly, neatly small.' ちんまりとした顔 describes a face that is small and neatly composed — endearing rather than critical. The ~とした pattern (mimetic + と + した) turns the mimetic into an attributive modifier. ~をしている with a body-part description means 'has [that kind of] face/figure.'