If u want better acceleration with superior road gripping capability, then go for 15" with 195/50/15 for your Saga LMST. U can opt for 55 series but if your car's suspension is stock, then these 55 series tyres will scrap your rear fenders if full loaded with rear passengers.

I've seen LMST with 17" but running with adjustables & man, it looked ganas betul.