Select <#2922#>[Buffers]<#2922#>
In the bottom half the actual amount of memory allocated to each buffer is displayed, if these values are not the same as the ones set by you, for some reason UniTerm was not able to use your configuration. In this case 5kB are reserved for system use, 2kB for the clipboard and the rest of available memory is allocated to the transfer buffer (up to the amount set by you, minimum 1kB), the remaining memory is used by the history buffer (min. 1kB).
The RS232 buffer values will only change if you save the value in a setup file and restart UniTerm, all other changes take effect immediately (and reset the buffers!).