There isnt a administration way to change it but you can change with css.

Add this code to your theme:

.google_map_field_display {
height: 300px;
width: 500px;
}

change height and width to fit the size you need.

Leaflet Widget for Geofield

Leaflet Widget for Geofield is a widget that provides a Leaflet based widget for adding and removing geometries (point, lineString, polygon) to a leaflet map and then have geofield store it.

Requirements

• Geofield (1.x)
• GeoPHP (1.x)
• Leaflet Widget
• Libraries (2.x)

Installation

Users that already have a geofield setup that is working can jump to (Install the widget).

1. Install geofield and geoPHP (see requirements).
2. Add a geofield to the content type you want to tag with geo-data. Choose WKT as a input widget and Geofield in the field display settings.
   
   Display settings
   

The Google Maps (GMap) module for Drupal integrates Google Maps with a variety of different systems. It provides both an API and integration with Views, as well as a few bundled sub-projects.

The primary user-facing feature of the GMap module is integration with Views. From the project README.txt:


The GMap module includes built-in integration with the Views module. You can use
this to turn any list of nodes, users, or other content into a Google Map that
plots each item on a map.

To make a GMap-based view, create or edit a view under Administer > Structure >
Views. Under the "Format" section, change the format by clicking on the current
format type (usually "Unformatted list"). Then change the format type to either
"GMap" or "Extended GMap".

For each view you create that uses a GMap format, you may either use a custom
configuration or inherit the site-wide defaults. To make an advanced map
configuration, you may type or copy/paste a GMap macro into the "Macro" textarea
at the top of the GMap format configuration settings. See the section on
"Macros" for more information about how to customize the configuration of your
map using these powerful tools. If you don't want a custom configuration for
this view, just leave the default value for the Macro field.

The only other critically important setting when configuring a GMap format in

DESCRIPTION:

The Geomap module will render a Google map in a block.

The locations placed on the google map are obtained by
analysing the current page for GEO microformat informaton.
When this info exists, a map will be rendered, when there
is no location information, no map will appear.

More information on GEO Microformats:
http://microformats.org/wiki/geo