This is a good question which we can answer by first looking into your site and into the requests made to the site itself.
If your website is an information site and the visitors will be pulling static content, then the resource usage will not be that big as the site content will be cached.
If your site is updated regularly with a lot of information/content (shop, blog and etc), the resource usage will be higher and it will differ each day. In cases like this it is vital to optimize your website (site level cache, database optimization and etc). You can also monitor the website on your current server and check the overall resource usage to make sure you will go with a droplet configuration that will match your needs.
The good thing about Digital Ocean’s droplets is that you can always resize the droplet’s resources in case you need more RAM or CPU in order to keep the website’s performance optimal.
My advise will be to check the resource usage on your current host/server and choose a droplet configuration close the resources used on your current server. Also if you’re happy with the performance of the site on the current server, you can deploy a droplet with the same specs (RAM, CPU) and if needed you can always upgrade the droplet in the future.
I hope this helps.