Clarification on Size of an Encryption Key Given a key with a certain key size (e.g., 64bits). Would I be correct to interpret this as 8 characters (since each character would be 8 bits under ASCII encoding)?

Because the ASCII scheme is limited to just 127 characters, this would mean that characters not included in the English language are not considered. Could it be instead that 64 bits has to be interpreted based on an encoding scheme such as unicode that includes many more characters? -- This would mean that 64 bits contains a much smaller number of characters since a unicode character takes more bits.
Yes, 64 bits is equivalent to 8 bytes, because there are 8 bits per byte.

"Could it be instead that 64 bits has to be interpreted based on an encoding scheme such as unicode that includes many more characters? "

No - the key does not need to be encoded as either ascii or UTF. Typically crypto algorithms operate either on an array of unsigned characters, or larger words when required (i.e. unsigned long long for 64 bit words).