How to set Semantic UI Dropdown size to match buttons, etc

Assuming your Dropdown has the button option set, you can pass the size you want in the className prop. For example:

<Dropdown text='Add Friend' icon='plus' labeled button className='icon tiny'> 

I think the right way should be wrap it inside a form, and apply the size classes to the form. The form could be a form tag, but also could be div:

<form className='ui form small'>
 <Dropdown>

or

<div className='ui form mini'>
 <Dropdown>

I think this is if you want to create the same size between dropdown and another component like button using size attribut, you can put the dropdown inside the button :

import React from 'react'
import { Dropdown, Menu, Button } from 'semantic-ui-react'

const options = [
  { key: 1, text: 'Choice 1', value: 1 },
  { key: 2, text: 'Choice 2', value: 2 },
  { key: 3, text: 'Choice 3', value: 3 },
]

const DropdownExampleSimple = () => (
  <div>
    <Button size="tiny" >
      <Dropdown text='Dropdown' options={options} simple item />
    </Button>
    <Button size="tiny">
      This is Button
    </Button>
  </div>
)

export default DropdownExampleSimple

this is the result :

Maybe can help you, thanks