つぶら (tsubura) means “round and cute”

More examples

森のリスがつぶらな瞳で木の実を見つめています。mori no ri su ga tsu bu ra na hitomi de ko no mi o mi tsu me te i ma su

A squirrel in the forest is gazing at a nut with round and cute eyes.

Presents a softly rounded appearance that makes a person or feature seem especially endearing.

まん丸manmaru: It describes something with a fully round, regular shape.
可憐karen: Having a delicate and charming quality in appearance.

目などが小さく丸く整っていて、かわいらしいさま。me na do ga chii sa ku maru ku totono tte i te ka wa i ra shi i sa ma

つぶら (tsubura, "round and charming") is a native Japanese adjective also written 円ら (tsubura), but its deeper origin is uncertain.

つぶらな瞳 (tsubura na hitomi, "round, charming eyes") usually describes a gentle, appealing look rather than a literal measurement of eye size.