TLegend *LegendInPiXel(TCanvas *c, int x1, int y1, int x2, int y2) {
   int w = gPad->GetWw();
   int h = gPad->GetWh();
   double x1ndc = (double)x1 / (double)w;
   double x2ndc = (double)x2 / (double)w;
   double y1ndc = (double)y1 / (double)h;
   double y2ndc = (double)y2 / (double)h;
   auto l = new TLegend(x1ndc, y1ndc, x2ndc, y2ndc);
   return l;
}

void legend_size_in_pixel() {
   auto c1 = new TCanvas("c1", "c1", 500,300);
   c1->Draw();
   auto l1 = LegendInPiXel(c1, 10, 10, 100, 30);
   l1->Draw();

   auto c2 = new TCanvas("c2", "c2", 0,350,900,500);
   c2->Draw();
   auto l2 = LegendInPiXel(c2, 10, 10, 100, 30);
   l2->Draw();
}