Openlayers map with dynamic markup
You can do that by passing a context function in Javascript that replaces the $-placeholders in your style object. I gave an example in this answer. The linked answer only changes the icon based on term rather than changing the marker size, but the principle's the same. Let me know if I can be of more help.

Edit: Sorry, I hadn't looked at your example when answering. That map doesn't involve clustering, so I think you can do it purely via the GUI. First you need to get the number as a field in Views and then use the appropriate placeholder (which you can see when you preview the data overlay display of the view) in the pointRadius field of the map's style.

Details


Make sure that the report count is a field in the OpenLayers Data Overlay display of your view.
Look at the preview of this display and note down the placeholder for the field - something like ${field_report_count}.
Go to Structure -> OpenLayers -> Maps and edit the appropriate map.
Go to Layers &amp; Styles, scroll down to OVERLAY LAYERS and look for the line which starts with the name of your view (it should end with - OpenLayers Data Overlay). Note down the name of the style in the STYLE column.
Go to Structure -> OpenLayers -> Styles and edit the style you noted down from step 4.
Put the field name you noted down from step 2. in the fields pointRadius and label. (I've never used the label field but I assume it does what you want.) You can also change the labelAllign, labelXOffset and labelYOffset fields if you want.