Steam – Put the created sticker into inventory CS GO

From either the Steam Store or the Steam interface, click the "Community" link, and you'll be taken you your "Steam feed." From here, the inventory link is on the right hand column, as shown below:

Note that you might have to change the tab, or check your Gifts in order to see everything there is to see in your inventory. The default is to display your TF2 items, whereas game gifts and so forth will show up in different parts of the interface:

In order to trade any items on Steam or use the Steam Market, you must have activated Steam Guard on your PC more than 15 days ago and own at least one non-free-to-play game.
There is an additional cooldown anytime you change your password or (in the case of the market) add a new payment method.

Once you are able to trade, the size of your Steam inventory is unlimited.

However, inventories for specific games can have additional restrictions. For example, non-premium TF2 users cannot trade items they drop or craft, and have a 50-slot limit to their TF2 inventory.

You can become a premium TF2 member by buying an item at the TF2 store, or by trading for an upgrade to premium gift. Your TF2 inventory size will then increase to 300, which can be increased up to 2000 using backpack expanders.

However, this new account only consists of 120 slots. If not mistaken, a regular account should have 720 slots?

The API you're using, IEconItems_570, is for retrieving your Dota 2 inventory. Until you buy a Dota 2 item from the Dota 2 store, your Dota 2 account is considered free-to-play. Once you buy an item, your account will become premium and your backpack size will increase.

To retrieve your Steam backpack instead, use Steam inventory REST call instead.