Storage capacity

The reason most probably is that you removed the Windows partition with Disk Utility instead of Boot Camp Assistant. Furthermore, I'm guessing your SSD was formatted using APFS.

If you have a backup for the macOS partition I suggest that you erase the SSD and then restore your backup.

Some good advice for the future: Never Do something on or to the computer unless you learn how to Undo it.

iCloud is not the place for your SSD files. An external HDD is better and more reliable. You will need one equal to or greater than the capacity of the SSD. You can use Time Machine or any other backup utility capable of making bootable clones. Here are a few to choose:

Backup Software

As an alternative, you can use any third-party backup utility your prefer. They all create straight file backups meaning you can restore individual files manually if you want by drag and drop copy. All are APFS compatible.

Recommended Backup Software

Carbon Copy Cloner

Get Backup

Deja Vu

SuperDuper!

Visit The XLab FAQs and read the FAQ on backup and restore. Also, read How to Back Up and Restore Your Files.

Install El Capitan or Later from Scratch

If possible back up your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed. Upon completion, you can restore your files from the backup. If you create a bootable clone of your SSD, then you can clone it back from the HDD to the SSD and skip the above installation procedure.

Clone Using Restore Option of Disk Utility

1. Restart the computer and immediately press and hold down the Command and R keys until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu then press the Continue button.
3. Select the destination volume from the left side list.
4. Select Restore from the Edit menu of Disk Utility. A panel will drop down.
5. Select the source volume from the drop down menu in the panel.
6. Double-check you got it right, then click on the Restore button.

Destination means an external drive. Source means the internal startup drive.

Restoring the clone is the same but you swap the Destination and Source devices.

I can't say. You could but then you would have no usable volume. The steps above will erase the volume which is all you need to do.