In your services.yml file you define the template for your edit Action:
app.admin.product:
class: AppBundle\Admin\ProductAdmin
arguments: [~, AppBundle\Entity\Product, AppBundle:Admin\Product]
tags:
- {name: sonata.admin, manager_type: orm, group: Products, label: Products}
calls:
- [ setTemplate, [edit, AppBundle:Product:edit.html.twig]]
In that template you can then override templates for fields in your form:
{% extends 'SonataAdminBundle:CRUD:base_edit.html.twig' %}
{% form_theme form.selectall 'AppBundle:Form:selectall.html.twig' %}
{% form_theme form.Country 'AppBundle:Form:country.html.twig' %}
Then my template looks like that:
{% block form_row %}
<div class="form-group">
{{ form_label(form) }}
{% set c = 0 %}
{% for i in form %}
{% set c = c+1 %}
{% if (c == 1) %}
<div style="float: left; width: 20%;">
{% endif%}
{{ form_row(i) }}
{% if ((c == 60) or (form|length == loop.index)) %}
</div>
{% set c = 0 %}
{% endif%}
{% endfor %}
</div>
{% endblock form_row %}
In this case, my countries check boxes appear in column of 60 elements, not in one column with the whole list of elements.
Hope this is helpful to someone else.