Dear Gryffindor Quidditch trainee,
Quidditch requires excellent broom-handling skills; the ability to maneuver whilst tracking the movements of fourteen players, three balls (four, for Seeker), and the referee; teamwork; and the ability to perform under pressure. All students are welcome to try out for House teams; however, first years seldom meet even the first of these qualifications.
It is my observation from my own years on the Gryffindor Quidditch team, as well as my years as professor and avid Quidditch fan, that size considerations matter most for Seekers, who, ideally, are slim and agile, as you point out. Beaters should be strong, but wiry or bulky matters less than overall power and precision. Chasers, arguably, come in the broadest range of body types. Nevertheless, body type is less important than you might think, compared to skillset.
Team captains are experienced enough to assess the potential of applicants at tryouts. For a student to turn down a position because it is not their preferred one would suggest lack of team spirit and, potentially, an inability to take coaching or direction. As that would suggest a deplorable attitude, it would likely be better for all involved for the student to step down, and leave the position to an individual with more loyalty and capacity for cooperation.
I expect such loyalty, team spirit, and cooperation from all members of the Gryffindor team.
And further, I expect to see the House Cup in my possession at the end of term. Good luck.
Hogwarts School of Witchcraft and Wizardry