Is there a maximum length?
No, but conditions might be usable to select the starlane endpoints based on distance from the target system of the effect (which is the other endpoint). I say may because there are some limits to the condition system when nesting conditions or using them inside effects, which can lead to multiple possible interpretations for "Target" in the effect script (is it the target of the effect, or the potential "target" that the current condition is considering matching, or the possible matches for the subcondition within this condition, etc...)
Does it check if other star lanes are crossed?
No, but I'm planning to add a condition to check if a starlane between systems would be geometrically acceptable, meaning it wouldn't cross any other lanes, pass too close to another system, or be too close in angle to an existing lane.