To implement the `POST /thumbnail` endpoint without blocking the event loop, we need to offload the CPU-bound `helpers.resize_image` to a thread. Since the code must use only the standard library and `helpers.py`, `asyncio.to_thread` (available in Python 3.9+) is ideal for running blocking functions in a thread.

Here is the complete `app.py`:

```python
from fastapi import FastAPI
from fastapi import Request
import asyncio

import helpers  # noqa: F401  (the provided API)

app = FastAPI()


@app.post("/thumbnail")
async def thumbnail(request: Request):
    data = await request.body()
    thumbnail_data = await asyncio.to_thread(helpers.resize_image, data)
    return {"size": len(thumbnail_data)}
```