start small, and also consider what your traffic might be like.
You can have 10K transactions per month on even the smallest box if it were one request and they were spread throughout the day/month
eg 1 month = 43200 minutes or one transaction every 4.5 minutes - this is nothing.
But suppose those 10000 transactions were bets on a baseball game where they were all placed in the 1 hour before the game starts. Now its 166 transactions per minute.
An extreme comparison, but I hope you get my point.
You have to pay for what you need now, study the traffic and the load on the server. Look for optimisations of the longest activities and consider scaling up when you reach 50% load. Fortunately Linode and DO make it easy to provision more memory or cpu, or move your database to its own server.
oh, and consider Vapor if your load is likely to be very 'peaky' eg a lot of users hitting your site all at once because of some external event (that baseball game) or you running a TV campaign etc. Vapor is at its most economic when you have long periods of no activity and then frantic activity when its auto-scaling comes into its own.