I am not sure the issue is the number of MPs, more the number of elected representatives overall, and the broken links between what we believe they do and what they actually do.
To give you an example, during an MPs Surgery, they might meet voters who talk about a range of things such as Defence, Fisheries, Economic Development, Waste Collection, Waste Disposal, Environmental issues, Council Tax collection, education.
How many of those is the MP directly responsible for? Well, Defence, Education and Environment are national government issues, Fisheries (thanks to CFP) is a European led issue, Economic Development and Waste Disposal are county level, whereas Waste Collection and Council Tax collection are borough level.
Although to be clear, the budget for fisheries is EU, Education is devolved locally from a central pot and for Shire and Borough spend the money comes 80% centrally (after money is collected and remitted locally) and 20% locally (which they are allowed to keep).
Looking after this we have 20,000 councillors and 650 MPs. Oh, that is ignoring Police and Crime Commissioners too.
It is a fascinating subject, and I think the real answer is that we should link accountability to direct election to tax raising authority to budgetary responsibility. Then we can work out whether we have the right number of MPs.