What vareity have you planted? The size of the fruit is primarily determined by the genetic make-up of the specific variety. So if big red tomatoes is the goal you first have to start with one of those varieties.

Then you have to factor in the growing conditions you provide the plant. In the ground vs. in a container, good nutrient levels, proper watering, cooperation from the weather, etc.

Then, and please understand that this claim is highly debated so there are no guarantees - to get a single bigger fruit from a cluster of blooms you supposedly pinch off all the blooms in that cluster except one.

But don't do that until after the small green fruit shows on the blooms. Otherwise you might pinch off the ones that were pollinated and keep the one that wasn't and end up with nothing. Ok?

So pick out a bloom cluster that has say 3-5 tiny green tomatoes on it and remove all but one.

Personally I have never found doing it to make a whole lot of difference when I have experimented. The fruit might be a little bigger but it doesn't gain much. But some of the grow-giant-tomato growers swear by doing it.

Dave

PS: check with the folks over on the Giant Vegetables forum to see if they have any other tips.

Besides the bloom issues...there is also the theory of pinching/pulling off the "sucker branches" for larger fruits. Again...hotly debated. Unless you are entering a contest...I would focus more on the other more important aspects of proper growing.

It is true that genetic of a variety determines fruit size, but there can be a lot of inconsistency in size due to the number of fruits growing all at the same time with a crowded foliage. So then reducing the burden on the plant may result in bigger fruits. People who grow tomatoes for championship do it all the time.
But To me even small fruit is fruit too. After all you cut them up when you use them.

There are folks who grow varieties for competition and they have their tricks, like using only fused blossoms, aka megablooms, and thinning out fruits, and more, and then there are most home growers who don't grow for competion who say thay want larger fruits.

In which case I agree with Dave,that if you want bigger fruits choose a variety that has bigger fruits.

Suckers. aka lateral branches, do not suck energy from the plant so that doesn't help to take them off for larger fruits,it just means less fruits on the plant since lateral branches are very productive as to setting fruits.

Let's say you have something like Rutgers, a medium sized red, and you want something larger, then think about Coustralee, Zogola, for red ones, maybe Omar's Lebanese or Large Pink Bulgarian for a pink one,justto give a couple of examples.

Carolyn