Hi
A good strategy is instead of creating one large module create few modules large enough to divide it on a domain (functionality) 
like
website, website_theme, e-commerce, portal extension in one module 
web, discuss, chat or backend related extension in one module
business logic like sale, crm, purchase extension in extension one module  

module size doesn't (much) affect the speed of the site. speed of the site depends on various parameters like business logic, deployment, catching mechanism, etc..

but if you divide module based on domain it will be very easy to manage and maintain the code

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