Why isn't the cost of shoes affected by their size? I have noticed that many shoe selling shops I have visited have the same price tag regardless of the shoe size.

Intuitively I'd think the bigger the shoe the more material and consequently capital invested in producing it. That being the case why would a manufacturer equally price dissimilar shoe sizes?
Most of the price of creating a shoe is the cost of labour to make it and the cost to ship it to the store. The cost of the materials needed to make the shoe is negligible. So, they can ignore the difference in material costs without losing too much money.

But why would they want to lose any money at all? Logistics. Keeping track of the different prices per shoe, printing the different labels, making sure the shoes are sold at the right price, paying people to determine how much of a price difference the company should charge are all things that cost money.

So the company is looking at creating a giant logistical nightmare for themselves, costing them who knows how much money, all for a penny difference between a size 8 and a size 13.

Trying to charge a different price would be a case of being 'penny smart; dollar stupid'.