go to your Cpanel extra css happy place..

and you can add li { line-height: 2em; }

where the 2em; is the spacing..

you could use pt or px.. whatever.. fiddle with it to your liking wink

now you can further narrow it down to just lists in posts..

by doing

Code
.post-content li {
 line-height: 2em;
}

which will only affect lists in posts wink

keep in mind this affects ALL lists in posts..