Making map less sniper-friendly?

more visual obstructions. The hydro temp fence/walls, or look at gravelpit cap point A (the huts). Or lots of corners and buildings.

For any other refence in sight lines, area sizes and attempts to cut down on sight lines, look at the Valve maps; which are found in the sdk_content folder under the names "sdk_prefix_mapname.vmf".

Some general area sizes can range between 1000 hammer units to 2500, and even ~3000 (for most A/D type maps).

As a general rule of thumb, there should be one or two good sniper spots, usually covering the quickest, most dangerous path. Make sure there is at least one good path that circumvents the sightlines though, as options like that are what make good maps fun. These side paths need to be designed such that they are not sniper friendly (many twists and turns, no long sightlines that snipers could use).

Also, as grazr said. In areas where there aren't clearly defined paths, but instead are just large battlegrounds, adding in obstructions such as huts, rocks, or little walls go a long way to reduce snipers' effectiveness (Gravelpit point A really is a perfect reference for this).

I don't think you should try to eliminate the sniper lines. Fix the sniper spots instead.
Sure, add cover where it's needed. But after awhile it will become too cluttered if the source of the problem isn't fixed.

range vs safety is a pretty decent trade off to keep in mind when dealing with snipers. If they have access to long sightlines, there needs to be a danger. like sideroutes or height disadvantage.
And as a opposite: If the sniper is hiding in a safe spot with health pickup, the overview/range of that spot needs to be limited.

If you build it, they will come.
- Fortress of Dreams

Sniping spots in Valve maps typically have flanking routes that can be used by spies/pyros to take out snipers (think RED side Goldrush stage 3, or pipeline, or sawmill.