The wisest course is to work out a 5 or 10 year cashflow, think about what kind of property the market wants to rent and figure out what you can really afford.
The main reason for buying a bigger place is to get greater exposure to the property market. If you have 100 grand in cash you will be about to get 500k's worth of exposure, because you can borrow on the strength of it. Big exposure is obviously a good thing if the property goes up in value, but it's not so good if the value stays stable or goes down. Like the man said, you have to decide 'do i feel lucky?'