Size comparison between

Goldcrest and Wren

Goldcrest bathing The Goldcrest is typically some 9 cm long (8.5-9.5 cm) and slimmer/lighter than the Wren.
Wren The Wren is typically some 9.5-10 cm long (9.0-10.5 cm) although its raised tail makes it look shorter. It is also tubbier and heavier.