The successful entrant will join four readers who are taking part in the website's 12-week Summer of Buff challenge.
Each participant will receive a free tailored diet and exercise programme, based on the goals they set. They will also benefit from ongoing one-to-one advice from Mansized fitness editor and personal trainer Anthony Punshon.
The website has run a number of challenges in the past and says that competitors who stick to the programme see visible results. Challengers will not have to spend hours at the gym, as the workouts typically last 40 minutes, three times a week.
Will Callaghan, editor of Mansized, said: "Personal trainers cost £60 an hour and more. Our challengers get free advice and barrow-loads of motivation from fellow readers."
Those taking part will be expected to post their progress on the site at least three times a week, and provide photographic evidence at monthly intervals.
Mansized is aimed at UK smart men aged from 25 to 40. It provides a mix of news headlines, reviews, answers from qualified experts, and message boards for discussion of all of the above.
To take part in the competition, contact Mansized.