Hi, sorry I don't understand?
If you change the size of your thumbnails to 800x600 and you set the size of detail window to same size (plus e.g. description or button area), e.g. to 820 x 660 (plus 60 for description or buttons), I see no problem there
Phoca Gallery doesn't set fix sizes for thumbnails nor for detail windows, maybe you mean the default sizes which are called if on parameters are saved. Every parameter in Joomla! must have some default value, in case no parameters were set, but it does not mean, the parameter is fix. If it will be fix, then it does not make any sense to list it in parameters.
Anyway there are two kinds of detail windows which have different behaviour:
https://www.phoca.cz/documents/50-phoca/ ... al-advices (8)
Jan