First of all you do not need to scale down your model when using a numerical simulation. You can find the real tsunami length by modifying your numerical model based on prototype scale.

However, if you scaled down your model, then we can track down the prototype of your tsunami size based on Froude similarity as follows. With 1:25 scale model and 20 seconds of overtopping the real (prototype) overtopping time will be  20 x 25 1/2 = 100 s. Assuming that all the positive part of tsunami overtopped your breakwater and if the depth near the breakwater is 10 m, your tsunami length (positive part) should be around 100 x (9.81x10)1/2  or approximately 1000 m or 1 km near the break water. At a depth of say 4000 m, your tsunami length (positive part) should be around 20 km indicating that your tsunami is probably a small one.

Thank you very much Pak Radianta Triatmadja, Matur nuwun.