Question

why size of neon is greater than size of flurine

Answer 1

Noble gases completely fill their valency shells. it is completely octet 
as the atomic radius of neon is greater than fluorine and noble gases have covalent radii

Answer 2

Thought the atomic size of elements decreases in a period from left to right, yet the radius of Ne is larger than that of F as Ne is a noble gas and it has van der wall's radius which is the largest among all the other types of radii.