As others have mentioned the functionality you're asking for can be found by making an iPart, and then creating a custom column or custom cell input as shown in the illustration below. Once a column or cell has been designated as a custom parameter, you can specify a range or increment (if needed). When placed in an assembly you are prompted to enter a size of your choosing at that time.
How to create a custom iPart (taken from the help files):
Create a standard iPart.
On one or more tabs in the iPart Author, determine the custom values to enter when the iPart is placed.
If the value is not already in the iPart table, select it in the left pane, and then click the Include arrow to add it.
In the iPart table, specify cells or columns in which to enter a custom value:
Right-click in a column and select Custom Parameter Column.
Right-click in any cell and select Custom Parameter Cell. Use this technique to create only one row that allows custom entry.
A custom column or cell is indicated by a blue background.
Save the file.
For more reading on custom iParts see the How do standard and custom iParts differ? topic at this link: