Red squirrels reach a maximum length of about 45cm (18 in.), of which up to 20cm (8 in.) may be the tail. Grey squirrels tend to be larger than Reds, reaching a maximum of 55cm (almost 2ft), of which 25cm (10 in.) may be tail. More commonly, Red squirrels reach about 21cm (8in.), while Greys attain about 26cm (almost 1ft) including the tail.
Red squirrels can weigh-in anywhere between 200 and nearly 500 grams (7 – 18 oz.), although they commonly attain weights of 280 to 300g (7 – 10.5 oz.) in Britain. There is some data from Belgium to suggest that the weight of Red squirrels may vary according to habitat, with higher weights reported—most notably during the winter months—from coniferous forests than in deciduous woodlands.
It appears that fat reserves are more seasonally variable in deciduous woodland than conifer forests because the winter food supply is more predictable in the latter. Males also tend to be heavier than females and body mass in general tends to be higher during the late autumn and winter and lowest in early summer. During the course of her PhD research, Sibylle Münch observed that Reds in Bavaria lost almost 10% of their body weight over winter.
Grey squirrels have been reported to weigh anywhere from 400 to 700 grams (14 – 25 oz.), with most specimens across Europe between 450 and 650g (16 – 23 oz.). In the UK, Grey squirrels average about 550g (19.5 oz.) and their weight is known to vary according to season, peaking during winter.