The right size will depend on your desires (vaginal gold anal), and your body shape. If you already have experience with a dildo, plug gold prostate stimulator, compare its size with these. That way you won't be completely in "unknown territory". Beginners will generally choose size S gold M.
Strap-on
Size S: insertable diameter 2.7 cm to 3 cm, insertable length 14 cm. Weight 245 g.
Size M: insertable diameter 3 cm to 3.7 cm, insertable length 16 cm. Weight 290 g.
Size L: insertable diameter 3.5 cm to 4 cm, insertable length 17.5 cm. Weight 345 g.
Bulb dimensions (identical for all 3 sizes): length 7 cm, width 4.1 cm.
Dildo
Size S: length 16.4 cm, diameter 3 cm.
Size M: length 18 cm, diameter 3.3 cm.
Size L: length 19 cm, diameter 3.45 cm.
Size XL: length 19.8 cm, diameter 3.6 cm.