Access properties of the parent with a Handlebars 'each' loop

The new method is using dot notation, the slash notation is deprecated (http://handlebarsjs.com/expressions.html).

So, the actual method to access to the parents elements are the following:

@root.grandfather.father.element
@root.father.element

In your specific example, you would use:

{{#each items}}
 <div style="font-size:{{@root.viewData.itemSize}}px">{{this}}</div>
{{/each}}

Another method from the official documentation(http://handlebarsjs.com/builtin_helpers.html) is using alias

The each helper also supports block parameters, allowing for named references anywhere in the block.

{{#each array as |value key|}}  
 {{#each child as |childValue childKey|}}
     {{key}} - {{childKey}}. {{childValue}}   
 {{/each}} 
{{/each}} 

Will create a key and value variable that children may access without the need for depthed variable references. In the example above, {{key}} > is identical to {{@../key}} but in many cases is more readable.

There are two valid ways to achieve this.

Dereference the parent scope with ../

By prepending ../ to the property name, you can reference the parent scope.

{{#each items}}
    <div style="font-size:{{../itemSize}}px">{{this}}</div>
    {{#if this.items.someKey}}
       <div style="font-size:{{../../itemSize}}px">{{this}}</div>  
    {{/if}}
{{/each}}

You can go up multiple levels via repeating the ../. For example, to go up two levels use ../../key.

For more information, see the Handlebars documentation on paths.

Dereference the root scope with @root

By prepending @root to the property path, you can navigate downwards from the topmost scope (as shown in caballerog's answer).

For more information, see the Handlebars documentation on @data variables.