This screen of the menu Options\Preferences lets you change the settings for reading and writing of messages in the groups in MesNews:

zone Reading:

Maximum number of headers to load: when selecting a group MesNews checks automatically if there are new messages. When this is the case they are automatically downloaded (at least the headers- subjects) when you fill in a value in the box MesNews will ask you for a validation if the group has more than the value (default 300 messages). When asked for validation, you can always change the amount. When the box Ask confirmation if > maxi lets you remove this validation.
Ask confirmation if > maxi : when ticked the validation screen, as explained above is shown. Otherwise all new messages are downloaded when you select a group, no matter how many messages there are available.
Maximum size of messages to be loaded: fill in this box the size of the messages to be downloaded automatically. (when filling in the value 0 you say that the sizes don't have to be checked). Attention : this value has an influence to all groups, selected and synchronized. To get a more specific setting you can use message rules .
Mark as read when leaving newsgroup : when ticked all messages will be marked as read when you leave a group.