How should the approach differ for product launches targeted towards large enterprises vs. ones for smaller startups in terms of timeline, activities, metrics?

Sales/partner/channel enablement is the biggest differentiator for enterprise-focused vs. startups or SMBs during the launch.

I'm coming from a software-as-a-service (SaaS) lens, which means that I have the option to include a variety of self-serve tactics that make it more scalable for smaller companies to get what they need during a launch.

Examples include:

• Product Guides: basic onboarding, core use cases, and templates so that small teams can see fast time-to-value

• Evergreen articles about practices, optimized for search (SEO), so that teams can use the tools and associated practices to be more successful

• Social media prompts, images, and links so that employees can share on social media (in our case, Product Managers and Engineers are part of our target market, so enabling these folks to share means that their network of similar users sees the information from a credible source)

• Community posts to engage directly with users

In contrast, many larger enterprises have a longer roll-out plan, choosing to engage with a sales rep for a discount, customer success for the implementation, and/or consultants for a phased roll-out. This means that the launch timeline needs to include extra time to brief sales and partners (assets might include new battlecards against the competition and/or the market landscape, decks with case studies/testimonials, updated demo environments, message house, etc.), and the deal cycle will take longer.

As for metrics, you can measure the increase in sign-ups and monthly active users for smaller accounts. You can measure the time-to-close, average deal size, and account expansion for larger enterprises.

I assume what you mean by your question is would there be a different approach for an enterprise company vs a startup. If this is the case it's a problem of different scale. The readiness coordination (internal) is going to much more complex than with a startup. There are lots of moving parts to consider. With a startup the window is generally shorter and easier to manage (notice I didn't say easy).

A rule of thumb I subscribe to is using the average length of the sales cycle to modulate your planning window. Longer sales cycle, longer window. You also have to factor into your planning how long it takes to get things done. Don't assume the day Dev says the product is ready you can train the sales team tomorrow. 

Hope that helps. 

Having worked on both the B2B and B2C markets, as David above says, the sales cycle length is completely different, and B2B can be weeks or months depending on the product and budget approval needed. 

With a startup you might be able to get to the decision maker or budget holder much faster than an enterprise, but depending on the product it might not be an essential buy for them depending on what stage they're in - make sure to factor in what Series / stage the startup is in to your planning.