doc: add Gemini image models sizes and aspect ratios reference

Documents supported aspect ratios and resolutions for:
- Gemini 2.5 Flash Image (1K only, 10 aspect ratios)
- Gemini 3 Pro Image (1K/2K/4K, 10 aspect ratios)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/gemini/sizes-and-ratios.md

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+# Gemini Image Models - Sizes & Aspect Ratios Reference
+
+Quick reference for Gemini image generation models supported by Banatie API.
+
+## Gemini 2.5 Flash Image
+
+**Model ID:** `gemini-2.5-flash-image`
+**Resolution:** 1K only
+**Default:** 1:1 (1024x1024)
+
+### Supported Aspect Ratios
+
+| Aspect Ratio | Pixels |
+|--------------|--------|
+| 1:1 | 1024 x 1024 |
+| 16:9 | 1408 x 768 |
+| 9:16 | 768 x 1408 |
+| 3:2 | 1216 x 832 |
+| 2:3 | 832 x 1216 |
+| 4:3 | 1152 x 896 |
+| 3:4 | 896 x 1152 |
+| 21:9 | 1536 x 640 |
+| 4:5 | 896 x 1088 |
+| 5:4 | 1088 x 896 |
+
+---
+
+## Gemini 3 Pro Image
+
+**Model ID:** `gemini-3-pro-image`
+**Resolutions:** 1K, 2K, 4K
+**Default:** 1K resolution
+
+### Size Options by Aspect Ratio
+
+| Size | Aspect Ratio | Pixels |
+|------|--------------|--------|
+| 1K | 1:1 | 1024 x 1024 |
+| 1K | 16:9 | 1408 x 768 |
+| 1K | 9:16 | 768 x 1408 |
+| 2K | 1:1 | 2048 x 2048 |
+| 2K | 16:9 | 2816 x 1536 |
+| 2K | 9:16 | 1536 x 2816 |
+| 4K | 1:1 | 4096 x 4096 |
+| 4K | 16:9 | 5632 x 3072 |
+| 4K | 9:16 | 3072 x 5632 |
+
+---
+
+## Banatie API Defaults
+
+- **Model:** Gemini 2.5 Flash Image
+- **Aspect Ratio:** 16:9 (1408 x 768)
+- **Resolution:** 1K (only option for 2.5 Flash)
+
+## Notes
+
+- Gemini 2.5 Flash is optimized for speed, supports only 1K resolution
+- Gemini 3 Pro supports higher resolutions (2K, 4K) for production use
+- All aspect ratios maintain approximately the same total pixel count per resolution tier