Dependence of carbocation stability with respect to the length of alkyl group
Carbocation stability is widely known to increase going from primary -&gt; secondary -&gt; tertiary.

In the absence of other factors (such as lone pair donation etc.), the major contribution to the greater stability of tertiary carbocations vs primary is due to hyperconjugation between the vacant p-orbital (carbocation) and the C-H sigma bonding MO's.

Based on hyper conjugation alone, you can predict that length of the alkyl chain will have no effect on the stability by itself, only the number of neighbouring C-H bonds with which hyper conjugation can take place.
In specific regard to ethyl vs n-propyl cation, neither are considered stable.