pre-allocating files

This follows a previous post for the following code:

http://www.mathworks.co.uk/matlabcentral/answers/23151-actxserver-cannot-read-all-of-the-data

    %using a faster method for importing excel
    clear all
    %obtain the name of each of the folders under investigation
    path_name='F:\University\CEH Lancaster\Project\Practice'; 
    TopFolder = path_name; 
    dirListing = dir(TopFolder); 
    dirListing=dirListing(3:end);%first 2 are just pointers

    for i=1:length(dirListing);
        SubFolder{i}=dirListing(i,1).name;
        SubFolderPath{i} = fullfile(path_name, dirListing(i,1).name);%obtain the name of each folder
        ExcelFile{i}=dir(fullfile(SubFolderPath{i},'*.xls'));%find the .xls files
        for j=1:length(ExcelFile{1,i});
            ExcelFileName{1,i}{j,1}=ExcelFile{1,i}(j,1).name;%find the name of each .xls file in each folder
        end
    end

    %manually obtain the name of the required worksheets
    Name_workbook={'WSB min', 'Bass min sond', 'Ble min', 'Est min'};

    %read data in from excel
    excel = actxserver('Excel.Application');
    excel.Visible=0;
    for i=1:length(ExcelFileName);
        a(i)=length(ExcelFileName{1,i});
        for j=1:a(i);
            file{1,i}{j,1}=excel.Workbooks.Open(fullfile(path_name,SubFolder{1,i},ExcelFileName{1,i}{j,1}));
            sheet1{1,i}{j,1}=excel.Worksheets.get('Item', Name_workbook(1,i));
            MyRange{1,i}{j,1}=sheet1{1,i}{j,1}.UsedRange; 
            MyData{1,i}{j,1}=MyRange{1,i}{j,1}.Value;        
        end
    end

If the number of Excel files in each cell (1x4 cell) ('ExcelFileName') is 19 19 19 18, how do I pre-allocate a cell array to store the data needed to be imported? pre-allocation is needed as the number of excel files is large.