This question inherently can't be answered as the problem is incompletely defined.
You have three very different problems based on where you are:
1) In space: Here it's simple: You want the tiniest engine you can get that doesn't sacrifice ISP. A bigger engine will complete the burn faster but so what? Using the same mass as fuel will almost certainly get you there faster by cutting travel time. Look at some of the burn times NASA lists for maneuvers by it's deep space probes and you'll see their engines have to be tiny.
2) On an airless body: Now it gets a lot more complex and there is no optimal answer without considering the whole mission. The higher the thrust of your engine the less delta-v you'll use but you'll be carrying more engine weight--and you'll also have had to get that engine weight there in the first place. Hauling that bigger engine might cost you more fuel than you save by reducing your gravity loss.
3) In atmosphere: Now it's even more complex as you have to consider air drag--now the bigger engine also carries an increased drag penalty as well as an increased weight penalty. Not only must you consider the whole mission but you must consider the shape of your rocket also.
Since cases #2 and #3 are optimizations between competing variables you have to optimize for each mission, there are no simple equations that give you an overall best answer.
Furthermore, unless you want to design an engine for the mission you're limited to what's on the shelf--and that may not be optimum for what you want to do. I play Kerbal Space Program and I've sent out plenty of overpowered spacecraft because the available parts don't match up perfectly with my objectives and because engines might be called upon to operate in multiple realms. Consider a simple mission: Land a probe on the Mun (the closest moon). The first part of the mission is atmospheric flight. The second part of the mission is space flight (transfer orbit, orbital insertion burn). The third part is airless body flight.
Now, the boosters used to get off Kerbin will generally fall back. Space flight is in my category #1--which would say to use the smallest engine. However, I'm going to land which means I must carry enough thrust to land. Why not simply use the landing engine for the space flight also? Thus I end up doing the space mission on engines capable of landing on the Mun, not merely getting there.
Now, consider a more expensive mission: Land on Ike. It's the moon of the Mars-analogue, a bit weaker gravity than the Mun. However, you have to do the interplanetary voyage. If it's a simple lander I would use the same system, just with a bit more fuel. However, if I'm taking something big (say, a manned rover capable of exploring the moon) the economics change: You get a smaller, cheaper rocket by doing all the space maneuvering with a nuclear engine. It doesn't come in a really small size, though, so it doesn't get used for the small stuff.