There's no specific field for monitor size in inches. We've added this to the wish list. Perhaps you could use a query like the one below to calculate the size based on the screen resolution and PPI values, which are scanned. Not sure how accurate this would be though.
Select Top 1000000 tsysOS.Image As icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.Username,
tblAssets.Userdomain,
tblAssets.IPAddress,
tblAssets.Firstseen,
tblAssets.Lastseen,
Ceiling(SqRt(Power((tblDesktopMonitor.ScreenWidth /
tblDesktopMonitor.PixelsPerXLogicalInch), 2) +
Power((tblDesktopMonitor.ScreenHeight /
tblDesktopMonitor.PixelsPerYLogicalInch), 2))) As MonitorSize
From tblAssets
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tblDesktopMonitor On tblDesktopMonitor.AssetID = tblAssets.AssetID
Where tblAssets.AssetID In (Select tblMonitor.AssetID
From tblMonitor) And tblAssetCustom.State = 1
Order By tblAssets.Domain,
tblAssets.AssetName