Android - How to select all messages Inbox Google?

The "Select All" feature is not available as of now in Inbox by Gmail app. Nor this feature of multiple selecting of spam emails at once available in the web version of Inbox by Gmail. Only Gmail app has the Empty Spam option but not select all. You can also swipe to delete in the new version of Inbox app. But that won't make a big difference if you have hundreds of emails spammed to you.

Take a look at the Features not implemented yet blog post section.

Gmail or Inbox by Gmail?

So, which one should you use? Most of the above can be reduced to just a couple key points for each product. But remember that: Inbox is not simply a user-interface change to Gmail, it as an entirely new product.

Gmail

• Customizable - multiple inbox formats and message organization options -
• Flexible - many message attributes to manage e-mail

Inbox by Gmail

• Simple - easy to learn and use
• Consistent - the same user-interface on all platforms

Of course there's no reason you can't use both. You could use Inbox on a day-to-day basis, or on mobile devices, and then use Standard Gmail for more complex activity like creating non-labeling filters, multiple message selection, emptying Trash and Spam, etc. Gmail and Inbox by Gmail are just tools and you should use whatever helps you manage your e-mail the best.

So feel free to continue to use Gmail, switch to Inbox, use both as appropriate, or use neither if you prefer your installed e-mail client. The choice is totally yours.[Ref.]

UPDATE : This functionality is now available in Inbox by GMail as of 10th Sep 2015. Google has added an option to delete all spam messages in single click via the "Empty Trash Now" button. Read here.

As posted on Google Plus,

Small update: You can now empty Trash and Spam in Inbox. Thanks to everyone for your feedback requesting this feature.

You can also do this in the webversion of Inbox by Gmail's spam page. Clicking on the "Empty Spam now" clears up the spam folder. Note that spam left undeleted for more than 30 days are deleted automatically.