diff --git a/core/pdf_exporter.py b/core/pdf_exporter.py
index 8ee5b85..c1461c9 100644
--- a/core/pdf_exporter.py
+++ b/core/pdf_exporter.py
@@ -507,6 +507,66 @@ def export_wireframe_to_pdf(
             stream_cmds.extend(lines_str[start_idx:start_idx + 5000])
             stream_cmds.append("S")
 
+    # 6. Size Scale Legend (Bottom Left, aligned directly on the bottom main grid lines with exact wireframe thickness)
+    stream_cmds.append("/GS100 gs")
+    x_grid_start_mm = grid_min_x
+    x_grid_end_mm = min(grid_max_x, grid_min_x + 10.0 * main_step)
+    num_main_steps = int(np.round((x_grid_end_mm - x_grid_start_mm) / main_step))
+
+    p_start_pt = to_pdf_pt(np.array([[x_grid_start_mm, grid_min_y]]))
+    p_end_pt = to_pdf_pt(np.array([[x_grid_end_mm, grid_min_y]]))
+
+    x_start_pt = p_start_pt[0, 0]
+    x_end_pt = p_end_pt[0, 0]
+    y_bar_pt = p_start_pt[0, 1]
+
+    # Exact same stroke thickness as object wireframe lines
+    stream_cmds.append(f"{line_width_pt:.2f} w")
+    stream_cmds.append("1 j 1 J")
+
+    # Main horizontal legend line directly overlapping bottom grid line
+    stream_cmds.append(f"{x_start_pt:.2f} {y_bar_pt:.2f} m {x_end_pt:.2f} {y_bar_pt:.2f} l S")
+
+    # Ticks directly overlapping vertical main grid lines
+    tick_cmds = []
+    for k in range(num_main_steps + 1):
+        xk_mm = x_grid_start_mm + k * main_step
+        pk_pt = to_pdf_pt(np.array([[xk_mm, grid_min_y]]))
+        xk_pt = pk_pt[0, 0]
+        tick_h_pt = (5.0 if k in (0, 5, num_main_steps) else 3.0) * MM_TO_PT
+        tick_cmds.append(f"{xk_pt:.2f} {y_bar_pt:.2f} m {xk_pt:.2f} {y_bar_pt + tick_h_pt:.2f} l")
+    stream_cmds.extend(tick_cmds)
+    stream_cmds.append("S")
+
+    # Proportionate font size scaling with linework thickness
+    font_size_pt = max(8.0, line_width_pt * 4.0)
+
+    stream_cmds.append(f"{sr:.3f} {sg:.3f} {sb:.3f} rg")
+    stream_cmds.append("BT")
+    stream_cmds.append(f"/F1 {font_size_pt:.1f} Tf")
+
+    for k in range(num_main_steps + 1):
+        xk_mm = x_grid_start_mm + k * main_step
+        pk_pt = to_pdf_pt(np.array([[xk_mm, grid_min_y]]))
+        xk_pt = pk_pt[0, 0]
+        tick_h_pt = (5.0 if k in (0, 5, num_main_steps) else 3.0) * MM_TO_PT
+        val_mm = k * main_step
+
+        if main_step >= 1000.0:
+            val_num = val_mm / 1000.0
+            unit = " m" if k == num_main_steps else ""
+            tick_str = f"{val_num:g}{unit}"
+        else:
+            unit = " mm" if k == num_main_steps else ""
+            tick_str = f"{val_mm:g}{unit}"
+
+        x_text_pt = xk_pt - (len(tick_str) * font_size_pt * 0.28)
+        y_text_pt = y_bar_pt + tick_h_pt + (2.5 * MM_TO_PT)
+        stream_cmds.append(f"1 0 0 1 {x_text_pt:.2f} {y_text_pt:.2f} Tm")
+        stream_cmds.append(f"({tick_str}) Tj")
+
+    stream_cmds.append("ET")
+
     stream_content = "\n".join(stream_cmds) + "\n"
     stream_bytes = stream_content.encode("utf-8")
 
@@ -516,7 +576,7 @@ def export_wireframe_to_pdf(
     page_header = (
         f"3 0 obj\n"
         f"<< /Type /Page /Parent 2 0 R /MediaBox [0 0 {page_w_pt:.2f} {page_h_pt:.2f}] "
-        f"/Resources << /ExtGState << /GS25 5 0 R /GS50 6 0 R /GS100 7 0 R >> >> "
+        f"/Resources << /ExtGState << /GS25 5 0 R /GS50 6 0 R /GS100 7 0 R >> /Font << /F1 8 0 R >> >> "
         f"/Contents 4 0 R >>\n"
         f"endobj\n"
     )
@@ -527,6 +587,7 @@ def export_wireframe_to_pdf(
     objects.append(b"5 0 obj\n<< /Type /ExtGState /CA 0.25 /ca 0.25 >>\nendobj\n")
     objects.append(b"6 0 obj\n<< /Type /ExtGState /CA 0.5 /ca 0.5 >>\nendobj\n")
     objects.append(b"7 0 obj\n<< /Type /ExtGState /CA 1.0 /ca 1.0 >>\nendobj\n")
+    objects.append(b"8 0 obj\n<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>\nendobj\n")
 
     header = b"%PDF-1.4\n%\xe2\xe3\xcf\xd3\n"