Which is correct grammar?

Which is correct grammar :I wish I was slimmer or I wish I were slimmer?

When you are using the simple past, you use 'was' for the 1st and 3rd persons singular, and you use 'were' for the other persons:
"I/He/She/It was here yesterday."
"We/You/They were here yestersay."

Now, in conditional sentences or in any other instances in which the subjunctive mood is needed (example, when you say "wish", you can use 'was' for the 1st and 3rd persons singular in informal English:
"If I was you, I'd tell her."
"I wish he was older."

Now, in formal English, and mainly in writing, you use 'were' for all persons.
"I wish you were here."
"I wish he were here."

When it comes to the subjunctive, you have a choice, but remember "appropriateness". If you're applying for a job or writing an academic paper -just to mention two examples- the more formal your style, the better.

So the short answer: use were.

thx editor

Were slimmer. For starters it just sounds better.