Size does matter, if you website is for a baker or a plumber, perhaps a 1 to 5 page website is all you need but if you service multiple areas of the United Kingdom, you may want to consider utilising landing pages per area for your SEO to really start working well for you. If you are selling products online, a small site might be right but with a large collection of products. And if you really want to ramp up you SEO Optimisation, additional pages are required for blog posts. Keep to your budget but consider the fact that, the more you can do for SEO, the less you'll need to pump into google ads/PPC. It's all about the long term investment.