45x35 suitcases how much can you really fit in

[Tremour]

need to book my flights as I am going away for 6-7 days to malta but can't decide if I should book extra luggage its additional £80 and the ticket is only £97.

I measured my current cabin bag and its about 5 cms bigger than 45cm but not as deep.

I've only ever done a weekend away with that size suitcase, feel like even I wore things twice it would be a struggle.

Even if you wore things twice? Travelling light means you wear things multiple times. You can rinse them out and let them dry if you think they are stale.
You won’t be able to use that bag anyway - the measurements are what matter, not the volume. A week in a warm country with cabin baggage is defo doable as long as you don’t want completely new outfits every day and evening.

I spent a week in Madrid taking a bag that size. Use compression cubes to help fit more in. Don’t just throw everything you own in, make a list of the outfits you want to wear. Plan to wear each item more than once.

so I have an event I am going to for 3 nights need outfits for that, then something to wear during the day which I can double on wearing those

If you have dressy events, pay for hold baggage. That’s not travel light territory really.