Here is the definition of Sylow p-group (source: wikipedia)
For a prime number p, a Sylow p-subgroup (sometimes p-Sylow subgroup) of a group G is a maximal p-subgroup of G, i.e. a subgroup of G that is a p-group (so that the order of every group element is a power of p) that is not a proper subgroup of any other p-subgroup of G. The set of all Sylow p-subgroups for a given prime p is sometimes written Sylp(G).
Does that mean it is possible that if I have $2$ sylow p-groups $P_1,P_2$ with $P_1 \neq P_2$ then
$|P_1| \neq |P_2|$ (if neither $P_1 \leq P_2,$ nor $P_2 \leq P_1)$ or am I misunderstanding the definition.