Regression of #16455 in v5 - Let custom fields specify a custom input size
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## Bug report

### Required System information

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- Node.js version: 22.12.0
- NPM version: 10.9.0
- Strapi version: 5.5.0
- Database: SQLite
- Operating system: Windows 11
- Is your project Javascript or Typescript: Typescript

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### Describe the bug

While porting my plugins from v4 to v5 I noticed that the behaviour described in #16455 seems to no longer be present

I have two plugins with Custom Fields which are showing different but incorrect results

#### Field 1

with the following configuration `strapi.customFields.register`
```json
{
    type: 'json',
    inputSize: {
        default: 6,
        isResizable: true
    }
}
```
I have the behaviour of the component not being able to be resized at all from the configuration 

![image](https://github.com/user-attachments/assets/19b74b52-c88a-4bf5-8011-6352a2b0bec1)


#### Field 2

with the following configuration `strapi.customFields.register`
```json
{
    type: 'string',
    inputSize: {
            default: 12,
            isResizable: true
    }
}
```
I have the behaviour of the component being able to be resized, but only up to 66%, 100% is not an option 

![image](https://github.com/user-attachments/assets/f9ff1e11-8b28-461f-af71-a5a3bf012ca6)

Both components with the same configuration allow for full resizing in my current v4 Strapi instance

As far as I can tell there are no changes listed to this API in the breaking changes list

