Exposing disk image size option to user

Nobody has claimed this yet.

I'm forwarding a problem I had when I build DMG with `electron-forge`, [which is said to be an appdmg limitation](https://github.com/electron-userland/electron-forge/issues/2133#issuecomment-763077551).

TL;DR: Can node-appdmg expose an option similar to `--disk-img-size` of [create-dmg](https://github.com/create-dmg/create-dmg) ??

Please see below.

### Issue Details

* **Electron Forge Version:**
* 6.0.0-beta.53
* **Electron Version:**
*
* **Operating System:**
* macOS 10.15.7
* **Last Known Working Electron Forge version:**:
* N/A

### Expected Behavior

I'd love to insert custom data into the app folder and have them included into the DMG and show up in the mounted folder.

### Actual Behavior

Trying to insert additional data into the output app folder and include it into the DMG, but after firing up a maker process, I saw a size calculation error that breaks the build

```sh
Making for the following targets: dmg
✖ Making for target: dmg - On platform: darwin - For arch: x64

An unhandled error has occurred inside Forge:
An error occured while making for target: dmg
Command failed: hdiutil create /var/folders/5j/ybqw096s7c162l_lgby0plw00000gn/T/N8YZNXR.dmg -ov -fs HFS+ -size 7313m -volname MAM-Hub
hdiutil: create failed - No space left on device
```
the size argument is incorrect and after digging into the source code of electron-forge, I found that this size is calculated internally and the `hdiutil` `-size` argument is not exposed.

### To Reproduce

- Create an Electron app and build it with `electron-forge`;
- Add a folder of data, in my case, another dependency PKG installer and its own supporting data, into the output folder;
- Build the app with yarn or npm;

You should see the similar error above.

### Additional Information

Here is my `package.json`

```json
{
"name": "myapp",
"productName": "myapp",
"version": "0.10.1",
"description": "my app",
"main": "src/main.js",
"scripts": {
"start": "electron-forge start",
"package": "electron-forge package",
"make": "electron-forge make",
"publish": "electron-forge publish",
"lint": "echo \"No linting configured\""
},
"keywords": [],
"author": {
"name": "me",
"email": "me@me.com"
},
"license": "MIT",
"config": {
"forge": {
"packagerConfig": {
"icon": "src/images/myicon",
"ignore": [
"/.gitignore",
"/.vscode",
"/yarn.lock",
"/node_modules/xyz/10.1.2/",
"/node_modules/xyz/build/",
"/node_modules/xyz/prebuilds/linux*"
]
},
"osxSign": {
"entitlements": "entitlements.plist",
"entitlements-inherit": "entitlements.plist",
"gatekeeper-assess": false,
"hardened-runtime": true,
"identity": "my id",
"signature-flags": "library"
},
"osxNotarize": "require:./notarize.js",
"hooks": "require:./hooks.js",
"makers": [
{
"name": "@electron-forge/maker-dmg",
"platforms": [
"darwin"
],
"config": {
"AdditionalDMGOptions": {
"code-sign": {
"identifier": "my certificate",
"signing-identity": "my cert id"
},
"contents": [
{
"name": "MAM-hub",
"path": "src/myapp/out/myapp-darwin-x64/myapp.app",
"type": "file",
"x": 370,
"y": 260
},
{
"name": "Applications",
"path": "/Applications",
"type": "link",
"x": 670,
"y": 260
},
{
"name": "My Dependency",
"path": "src/mam-hub/out/mam-hub-darwin-x64/mydep",
"type": "file",
"x": 240,
"y": 260
},
{
"name": "INSTALL.html",
"path": "../../temp/INSTALL_en.html",
"x": 240,
"y": 400
},
{
"name": "安装指南.html",
"path": "../../temp/INSTALL_zhs.html",
"x": 370,
"y": 400
}
],
"window": {
"position": {
"x": 200,
"y": 120
},
"size": {
"height": 660,
"width": 1020
}
}
},
"background": "../../res/installer/macos/background-orange.png",
"format": "UDZO",
"icon": "src/images/myapp.icns",
"iconSize": 90,
"name": "myapp",
"overwrite": true
}
},
{
"name": "@electron-forge/maker-wix",
"platforms": [
"win32"
],
"config": {
"arch": "x64",
"appUserModelId": "com.me.myapp",
"description": "description of myapp",
"exe": "myapp",
"iconPath": "src\\images\\me.ico",
"language": 1033,
"manufacturer": "me",
"name": "my app",
"programFilesFolderName": "myapp",
"shortName": "myapp",
"shortcutFolderName": "myapp",
"ui": {
"chooseDirectory": true,
"images": {
"background": "C:\\Temp\\background-orange.png",
"banner": "C:\\Temp\\banner-orange.png"
}
},
"version": "1.0.0"
}
}
]
}
},
"dependencies": {
"buffer-from": "^1.1.1",
"zeromq": "^6.0.0-beta.6"
},
"devDependencies": {
"@electron-forge/cli": "6.0.0-beta.53",
"@electron-forge/maker-dmg": "6.0.0-beta.53",
"@electron-forge/maker-wix": "6.0.0-beta.53",
"electron": "10.1.2"
}
}
```

The issue shows the failing hdiutil create invocation and the maker-dmg AdditionalDMGOptions configuration; start by tracing how node-appdmg assembles that command. Compare the requested --disk-img-size behavior with the current -size handling, then verify that a configured image size is passed through and prevents the reported no-space failure.

Written by the indexing model from the issue text.