The image file sizes we have on Unsplash are the only sizes we have available.
We have a handy '5-Minute Guide to Image Quality' that answers the difference between resolution, compression, and DPI, so that you can make sure you’re using the right image for the right use.
You can find it here: https://medium.com/unsplash/the-5-minute-guide-to-image-quality-ad7c3503c845
If you still require a larger file size you could try reaching out the the photographer directly. Should you wish to contact a photographer, you can do so via the ‘message’ button on their profile.
Please note, you will need to have an Unsplash account, and be logged in to access this feature.