diff --git a/openspec/changes/arena-size/.openspec.yaml b/openspec/changes/arena-size/.openspec.yaml
new file mode 100644
index 0000000..c0ad5ed
--- /dev/null
+++ b/openspec/changes/arena-size/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-11-08
diff --git a/openspec/changes/arena-size/design.md b/openspec/changes/arena-size/design.md
new file mode 100644
index 0000000..cb5cdbf
--- /dev/null
+++ b/openspec/changes/arena-size/design.md
@@ -0,0 +1,31 @@
+# Design
+
+## Context
+
+Output is already bound and a listener exists, but mode dimensions are ignored. See proposal.md.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Store output mode width/height.
+- Compute default logical size = 0.8 * mode / scale.
+- Keep grid resize/preserve behavior.
+
+**Non-Goals:**
+- Fullscreen mode.
+- Dynamic output hotplug handling beyond first output.
+
+## Decisions
+
+### D1: Output mode storage
+- Store `output_width`, `output_height` from `wl_output.mode`.
+
+### D2: Default size computation
+- After roundtrip, if output info available, set logical size; else fallback 1024x576.
+
+### D3: Fallback
+- 1024x576 logical (~80% of 1280x720) when no output info.
+
+## Open Questions
+
+None.
\ No newline at end of file
diff --git a/openspec/changes/arena-size/proposal.md b/openspec/changes/arena-size/proposal.md
new file mode 100644
index 0000000..36a755a
--- /dev/null
+++ b/openspec/changes/arena-size/proposal.md
@@ -0,0 +1,27 @@
+# Proposal
+
+## Why
+
+The current default arena is 400×400 logical pixels (50×50 cells), too small for comfortable exploration. The arena should occupy about 80% of a FullHD screen, adapting to the actual output size and scale.
+
+## What Changes
+
+- Use `wl_output` mode dimensions to compute a default logical window size equal to 80% of the output, divided by the active scale.
+- Keep the 8 px logical cell size; grid dimensions derive from the logical window size.
+- Preserve resizable behavior and grid recalculation on resize.
+- Provide a sensible fallback when output info is unavailable.
+
+## Capabilities
+
+### New Capabilities
+
+- `arena-sizing`: output-aware default arena size (~80% of screen) with fallback and resize handling.
+
+### Modified Capabilities
+
+None.
+
+## Impact
+
+- Affected code: `wlgol.c` (output mode handling, default size calculation, canvas/grid resize).
+- No new dependencies.
\ No newline at end of file
diff --git a/openspec/changes/arena-size/specs/arena-sizing/spec.md b/openspec/changes/arena-size/specs/arena-sizing/spec.md
new file mode 100644
index 0000000..30252e3
--- /dev/null
+++ b/openspec/changes/arena-size/specs/arena-sizing/spec.md
@@ -0,0 +1,42 @@
+# Spec Delta
+
+## Purpose
+
+Defines how the arena default size is derived from the output and how resizing affects the grid.
+
+## ADDED 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
\ No newline at end of file
diff --git a/openspec/changes/arena-size/tasks.md b/openspec/changes/arena-size/tasks.md
new file mode 100644
index 0000000..48575ae
--- /dev/null
+++ b/openspec/changes/arena-size/tasks.md
@@ -0,0 +1,11 @@
+# Tasks
+
+## 1. Output handling
+
+- [ ] 1.1 Store output mode dimensions in the output listener.
+- [ ] 1.2 Compute default logical window size as 80% of output divided by scale, with fallback.
+
+## 2. Verification
+
+- [ ] 2.1 Build and run, verifying window is ~80% of screen.
+- [ ] 2.2 Resize window and verify grid updates.
\ No newline at end of file