Keep in mind that hundreds of millions of years of extinctions and adaptive radiations have taken place between then and now. It's not only a question of whether insects can exist at the larger sizes they once grew to, but also whether they'd be competitive enough to take back their former ecological niches from the current incumbents. To use a tech analogy, it's like asking whether a flip phone can dominate the premium phone market in 2014, because they once dominated it in 2005.
Judging from the range and distribution of insects alive in the present, it seems that for most insects, small size and rapid reproduction are the way to go for them. Their body plan, with it's less efficient circulatory system, and less complex brains, seem to put them at a disadvantage if they began to grow larger. Remember that there are a vast array of mammals and birds that specialize specifically in preying on insects.