A quick fix for this is to add the following entry to the ReportingServices.css file.
.DocMapAndReportFrameYou can usually find the ReportingServices.css file at: C:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportManager\Styles, however your location may be different depending on your installation.
{
min-height: 800px;
min-width: 800px;
}
peace