Like will it slow down your boot-up?
Also what problems will arise if you allocate more hard drive memory for virtual memory than what is available, will that cause any issues?
Allocating extra virtual memory should not slow down boot up, because these page files (on most OSes; you didn't specify) are created once and then just sit there until they are used. They are just a preallocated block.
The only performance delay will be if the OS decides to use them because it has filled all the real memory (RAM), in which case you will experience slowness as your system pages to disk.
If you over-allocate, then your system will behave as if it has more RAM, albeit with this performance penalty that disks are slower than actual RAM.
hope that helps.
If you mean the page files, then, besides taking up space on the hard drive, there are no performance penalties associated with large page files. They will just go underused, and you'll waste space better used for pictures of funny cats or what have you.
The memory management unit manages the virual memory. It translates virtual memory adresses to actuall physical adresses. The bigger the virtual memory space, the bigger the adress table becomes in which is written, which virtual adress belongs to which physical adress. A big table can theoreticaly result in slower translation of the adresses and therefore in slower reading and writing speeds. I can't quantify this effect. Its probably not relevant.
From what I understand, since windows 7 and later, they've gotten good at letting Windows determine the page file size, so I would check the box to do it automatically.
Aside from Virtual Memory being slower than RAM another issue is slower i/o speed to the hdd (which is often the bottleneck in performance anyway) and you run the risk of burning through a hdd much faster.