change cellsize in an object of class SpatialGridDataFrame

Dear all,

I'd like to change size of cells in an object of class 
SpatialGridDataFrame. How can I do this?
The object I manipulate is summarized below. As you can see, size of 
cell lightly differ between latitude and longitude.

Many thanks for your help.

max


summary(CCSMpastgrd)
Object of class SpatialGridDataFrame
Coordinates:
            min      max
long -4.437526 29.97919
lat  40.062441 54.97922
Is projected: FALSE
proj4string : [+proj=longlat]
Number of points: 2
Grid attributes:
      cellcentre.offset   cellsize cells.dim
long         -4.416667 0.04171723       825
lat          40.083333 0.04178371       357
Data attributes:
       Min.    1st Qu.     Median       Mean    3rd Qu.       Max.       
NA's
3.7500e-08 8.8200e-06 3.8300e-05 5.3880e-04 1.9390e-04 4.5430e-02 
5.8586e+04

Maxime Pauwels
Researcher ID: http://www.researcherid.com/rid/A-1745-2009

Equipe G?n?tique&  ?volution de l'adaptation aux environnements pollu?s par les m?taux lourds
Laboratoire de G?n?tique et Evolution des Populations V?g?tales
FRE CNRS 3268
B?timent SN2
Universit? de Lille, USTL-Lille1
http://gepv.univ-lille1.fr
Tel  : +33 3 20 33 62 38
Fax  : +33 3 20 43 69 79