Would it be possible to have a minimum size for the displayed ping?

..and more or less off-topic, is it possible to build the current source against 1.7.10?

That is intended behavior, and can be changed in the config.
There is a 1.7.10 branch that is possible to build yes, but the latest source is not possible to build against for 1.7.10 no.

But there is no setting for size and it's not fetched in the code anywhere as far as i saw?

Oh I misunderstood what you said.
The distance at which you can "set" a ping can be changed in the config. As for the distance you can see a Ping, is only limited by your render distance.

Yeah of course. It's more a problem of scaling, i set up the maximum distance already.
But at a distance where you just want to ping "i am here!" its like a single pixel or so..