diff --git a/openspec/changes/arena-size/.openspec.yaml b/openspec/changes/archive/2026-11-08-arena-size/.openspec.yaml
similarity index 100%
rename from openspec/changes/arena-size/.openspec.yaml
rename to openspec/changes/archive/2026-11-08-arena-size/.openspec.yaml
diff --git a/openspec/changes/arena-size/design.md b/openspec/changes/archive/2026-11-08-arena-size/design.md
similarity index 100%
rename from openspec/changes/arena-size/design.md
rename to openspec/changes/archive/2026-11-08-arena-size/design.md
diff --git a/openspec/changes/arena-size/proposal.md b/openspec/changes/archive/2026-11-08-arena-size/proposal.md
similarity index 100%
rename from openspec/changes/arena-size/proposal.md
rename to openspec/changes/archive/2026-11-08-arena-size/proposal.md
diff --git a/openspec/changes/arena-size/specs/arena-sizing/spec.md b/openspec/changes/archive/2026-11-08-arena-size/specs/arena-sizing/spec.md
similarity index 100%
rename from openspec/changes/arena-size/specs/arena-sizing/spec.md
rename to openspec/changes/archive/2026-11-08-arena-size/specs/arena-sizing/spec.md
diff --git a/openspec/changes/arena-size/tasks.md b/openspec/changes/archive/2026-11-08-arena-size/tasks.md
similarity index 100%
rename from openspec/changes/arena-size/tasks.md
rename to openspec/changes/archive/2026-11-08-arena-size/tasks.md
diff --git a/openspec/specs/arena-sizing/spec.md b/openspec/specs/arena-sizing/spec.md
new file mode 100644
index 0000000..a242a39
--- /dev/null
+++ b/openspec/specs/arena-sizing/spec.md
@@ -0,0 +1,41 @@
+# arena-sizing Specification
+
+## Purpose
+Defines how the arena default size is derived from the output and how resizing affects the grid.
+
+## Requirements
+
+### Requirement: Default arena is about 80% of output
+The system SHALL compute the default logical window size as 80% of the output mode dimensions divided by the active scale.
+
+#### Scenario: Output available
+- **WHEN** output mode and scale are known at startup
+- **THEN** the default logical window size is approximately 80% of the output
+
+### Requirement: Fallback when output unknown
+The system SHALL use a fallback default size when output info is unavailable.
+
+#### Scenario: No output info
+- **WHEN** output mode is not reported
+- **THEN** a fallback logical size is used
+
+### Requirement: Cell size remains 8 logical pixels
+The system SHALL keep an 8 logical pixel cell size.
+
+#### Scenario: Grid dimensions
+- **WHEN** the logical window size is known
+- **THEN** grid columns and rows are logical size divided by 8
+
+### Requirement: Resizable window
+The system SHALL allow the window to be resized by the user.
+
+#### Scenario: Resize
+- **WHEN** the user resizes the window
+- **THEN** the grid dimensions update accordingly
+
+### Requirement: Grid preserved on resize
+The system SHALL preserve overlapping cells when the grid dimensions change.
+
+#### Scenario: Enlarge
+- **WHEN** the window grows
+- **THEN** existing cells within the old bounds remain