Why do humans grow taller than their ancestors?
A likely misunderstanding of yours

Now we almost don't fight with other species

Misunderstanding about selection
As you will go through this course, you will understand why this sentence makes little sense. A change in allele frequency via natural selection is caused by a fitness differential among genotypes within a population. The existence of a competitor species would eventually affect this fitness differential but there is no need for a competitor for having a selection pressure.
I think you are having the view that selection is a process that allows a species to be better at coping with a competitor but this is wrong. Selection will increase the frequency of individuals having high fitness in the population. The resulting population may well not be a better competitor.
False opinion that we are not competing with other species
Also, about nearly 50,000 people are dying every day from infectious disease (UN press release). So, we are definitely still &quot;fighting&quot; with other species. Adaptation to malaria in humans is a famous case study (see this answer and its links).
Also, according to reuters (who fail to link to their source), the worldwide cost of pests in agriculture is estimated to about 540 billions USD per year! So yep... competition there is.
What selection pressures are there on human height?
Review paper recommendation
Stulp and Barrett 2016 is a review paper on the evolution of height in humans that you might want to have a look to have a clearer picture than what I will give here. It is easy to read. I highly recommend having a look at it.
They cite a lot of papers. I won't cite them again below but I will just attempt to make a vague summary of the parts which I think will interest you the most.
What selection pressures are there on human height?
The question appears to not be that easy to answer. In humans, height is highly heritable and there is a lot of variance in height among populations. It is likely that different selection pressures may have acted on different populations.
There are a diversity of selection patterns
In short, selection pressures on height include intra and intersexual selection. Taller adults seem to have a reduced mortality rate. Taller women have a lower risk of complications when giving birth. However, in both men and women, the correlation between height and number of offspring varies a lot from study to study (and likely from population to population). In the USA for example, men having the highest number of offspring are of intermediate height (balancing selection).
It appears that epigenetic variation also have an important impact on height Simeone and Alberti 2014 and there might have a sex-driven intra-locus conflict on height (Gilks et al. 2014; Stulp and Barrett 2016)
There is evidence of current selection on height
We have a fair number of evidence that height was and is still under selection in humans but we think that the exact pattern of selection pressures acting on it is quite complex. Note that there is evidence that height is still under selection today.