Quite a bit of guess-work going on here regarding your GUI and what it's doing but, ... (see comments)
function file_Callback(hObject, eventdata, handles)
[Filename,Pathname] = uigetfile('*.txt','Select Text File');
if isequal(Filename,0)
disp('User selected Cancel')
else
handles.filename_txt = fullfile(Pathname, Filename); % use fullfile() to combine path and filename
set(handles.text_path,'String',handles.filename_txt);
fr = fileread(handles.filename_txt); % fileread is easier to implement and meets your needs
handles.filesize = numel(fr); % simply count characters (numeric output) not bytes
set(handles.text_size,'String',num2str(handles.filesize)); % Convert number -> char
guidata(hObject,handles); % Update handles at the end
end
function Encode_Callback(hObject, eventdata, handles)
if handles.filesize < 33 % no need for length()
handles.textLabel = sprintf('Message to short');
set(textLabel, 'String', textLabel); % ??????? no idea what this is/does
return;
end
end
end
