It's not so much the size of the boat but how easy it is to keep under control and the 18 is a pretty good boat for this when sailed as intended ie two up.
However if you are learning you should be doing it with a sailing club under controlled conditions with support staff and preferabley instruction.

Unless very experienced, which you don't seem to be,you should not be taking a two man boat out by yourself.

If you are going to sail by yourself you would be much better learning on a 14ft boat as it can be handled more easily in a capsize.
Safety has to be the first choice, especially for people
that have little experience to draw on in an emergency.

Just the way I see it.

Regards,
Phill