diff --git a/builder/main.py b/builder/main.py
index 0c97670..1c8f997 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -144,12 +144,27 @@ def _update_max_upload_size(env):
         if p["type"] in ("0", "app")
     }
 
-    # One of the `factory` or `ota_0` partitions is used to determine available memory
-    # size. If both partitions are set, we should prefer the `factory`, but there are
-    # cases (e.g. Adafruit's `partitions-4MB-tinyuf2.csv`) that uses the `factory`
-    # partition for their UF2 bootloader. So let's use the first match
+    partitions = {p["name"]: p for p in _parse_partitions(env)}
+
+    # User-specified partition name has the highest priority
+    custom_app_partition_name = board.get("build.app_partition_name", "")
+    if custom_app_partition_name:
+        selected_partition = partitions.get(custom_app_partition_name, {})
+        if selected_partition:
+            board.update("upload.maximum_size", _parse_size(selected_partition["size"]))
+            return
+        else:
+            print(
+                "Warning! Selected partition `%s` is not available in the partition " \
+                "table! Default partition will be used!" % custom_app_partition_name
+            )
+
+    # Otherwise, one of the `factory` or `ota_0` partitions is used to determine
+    # available memory size. If both partitions are set, we should prefer the `factory`,
+    # but there are cases (e.g. Adafruit's `partitions-4MB-tinyuf2.csv`) that uses the
+    # `factory` partition for their UF2 bootloader. So let's use the first match
     # https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-guides/partition-tables.html#subtype
-    for p in  _parse_partitions(env):
+    for p in partitions.values():
         if p["type"] in ("0", "app") and p["subtype"] in ("factory", "ota_0"):
             board.update("upload.maximum_size", _parse_size(p["size"]))
             break