```python
from fastapi import FastAPI
import helpers
import asyncio

app = FastAPI()

@app.post("/thumbnail")
async def thumbnail(data: bytes):
    # Run the CPU-bound resize_image in a thread pool to avoid blocking the event loop
    thumbnail_bytes = await asyncio.get_event_loop().run_in_executor(
        None, helpers.resize_image, data
    )
    return {"size": len(thumbnail_bytes)}
```