```python
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
import asyncio

import helpers  # noqa: F401  (the provided API)

app = FastAPI()


@app.post("/thumbnail")
async def thumbnail(request: Request):
    data = await request.body()

    loop = asyncio.get_running_loop()
    thumbnail_bytes = await loop.run_in_executor(None, helpers.resize_image, data)

    return JSONResponse(content={"size": len(thumbnail_bytes)}, status_code=200)
```