If you want up 12R (12"x18") sizes, most of the Kodak labs with the new Noritsu machines will handle it. Anywhere larger you might try Fotohub at Raffles Place or the labs with access to a Durst Lambda. the latter can print up to paper widths of 48" or 4 feet. Max length around 70-80" I believe.
// Technically Lambdas are rated to print up to 50" width..... but that's a risk I rather not take so give a 1 inch allowance on both edges of the paper //